Method of making dielectrically isolated integrated circuit

ABSTRACT

General purpose methods for the fabrication of integrated circuits from flexible membranes formed of very thin low stress dielectric materials, such as silicon dioxide or silicon nitride, and semiconductor layers. Semiconductor devices are formed in a semiconductor layer of the membrane. The semiconductor membrane layer is initially formed from a substrate of standard thickness, and all but a thin surface layer of the substrate is then etched or polished away. In another version, the flexible membrane is used as support and electrical interconnect for conventional integrated circuit die bonded thereto, with the interconnect formed in multiple layers in the membrane. Multiple die can be connected to one such membrane, which is then packaged as a multi-chip module. Other applications are based on (circuit) membrane processing for bipolar and MOSFET transistor fabrication, low impedance conductor interconnecting fabrication, flat panel displays, maskless (direct write) lithography, and 3D IC fabrication.

This is a divisional of application Ser. No. 07/865,412, filed Apr. 8,1992, U.S. Pat. No. 5,354,695.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for fabricating integrated circuits onand in flexible membranes, and to structures fabricated using suchmethods.

2. Description of Related Art

Mechanically and thermally durable free standing dielectric andsemiconductor membranes have been disclosed with thicknesses of lessthan 2 μm. (See commonly invented U.S. Pat. No. 4,924,589, and U.S.patent application Ser. No. 07/482,135, filed Feb. 16, 1990, now U.S.Pat. No. 5,103,557, both incorporated herein by reference). Thisdisclosure combines the novel use of these technologies and otherintegrated circuit (IC) processing techniques to form ICs as membranestypically less than 8 μm thick. This approach to IC farication fallsunder the generic industry-established title known as DielectricIsolation (DI), and is inclusive of subject areas such asSilicon-on-Insulator (SOI) and Silicon-on-Sapphire (SOS). ICs formedfrom dielectric and semiconductor membranes can reduce significantly thenumber and complexity of processing steps presently used to providecomplete IC device isolation; dielectric isolation techniques thatprovide dielectric isolation on all surfaces of the individual circuitdevices comprising the complete IC are not as yet widely used in volumeIC fabrication. Integrated Circuits are defined as commonly understoodtoday when referring to SSI, MSI, LSI, VLSI, ULSI, etc. levels ofcircuit complexity.

SUMMARY OF THE INVENTION

This invention is directed to a general method for the fabrication ofintegrated circuits and interconnect metallization structures frommembranes of dielectric and semiconductor materials. The fabricationtechnology in accordance with this invention is referred to herein asMembrane Dielectric Isolation (MDI), and the circuits made from it ascircuit membranes. The novel use of materials and processing techniquesprovides for the fabrication of high temperature, mechanically durable,large area free standing membranes (greater than 1 square cm in area)from low stress dielectric and/or semiconductor films. These membranespermit the application (continued use) of most of the establishedintegrated processing methods for the fabrication of circuit devices andinterconnect metallization.

In accordance with the invention, an integrated circuit is formed on atensile low stress dielectric membrane comprised of one layer or apartial layer of semiconductor material in which are formed circuitdevices and several layers of dielectric and interconnect metallization.Also, a structure in accordance with the invention is a tensile membraneof semiconductor material in which are formed circuit devices withmultiple layers of tensile low stress dielectric and metallizationinterconnect on either side of the semiconductor membrane.

The membrane structure is a processing or manufacturing structure forenabling the manufacture of novel and more cost effective integratedcircuits. This is in addition to an objective to manufacture anintegrated circuit, or portion thereof, in a membrane or thin film form.

The general categories of circuit membranes that can be made by thisinvention are:

1. Large scale dielectric isolated integrated circuits formed on or fromsemiconductor or non-semiconductor substrates.

2. Multi-layer interconnect metallization circuits formed on or fromsemiconductor or non-semiconductor substrates.

The primary objectives of the MDI fabrication technology disclosedherein are the cost effective manufacture of high performance, highdensity integrated circuits and integrated circuit interconnect with theelimination or reduction of detrimental electrical effects on theoperation of individual circuit devices (e.g. diodes, transistors, etc.)by completely isolating with a dielectric material each such circuitdevice from the common substrate upon which they are initiallyfabricated, and therefore, from each other, and to provide a moreversatile and efficient physical form factor for the application ofintegrate circuits. Some of the benefits of the MDI IC fabricationprocess are the elimination or reduction of substrate current leakage,capacitive coupling and parasitic transistor effects between adjoiningcircuit devices. The MDI IC fabrication process benefits extend toseveral other categories of IC fabrication such as lower IC processingcosts due to fewer IC isolation processing steps, greater IC transistordensities through the capability to use established IC processingtechniques to fabricate interconnect metallization on both sides of aMDI IC circuit membrane, and greater IC performance through noveltransistor structures.

The strength of the MDI processes is primarily drawn from two areas:

(1) The ability to make a large area flexible thin film free standingdielectric membrane, typically framed or suspended or constrained at itsedges by a substrate frame or ring, or bonded frame or ring. Thismembrane is able to withstand a wide range of IC processing techniquesand processing temperatures (of at least 400° C.) without noticeabledeficiency in performance. The present dielectric materials that meetthese requirements are silicon dioxide and silicon nitride films whenprepared with specific low stress film deposition recipes for instanceon equipment supplied by Novellus Systems, Inc. Dielectric free standingfilms created by CVD process methods such as silicon carbide, boronnitride, boron carbon nitride aluminum oxide, aluminum nitride, tantalumpentoxide, germanium nitride, calcium fluoride, and diamond have beenproduced, and can potentially be used as one of the dielectric materialsin a MDI circuit membrane when deposited at an appropriate level ofsurface stress. Advances in the technology for making low stressdielectric films will likely produce additional free standing films thatcan be used as described herein.

(2) The ability to form a uniform thin film single crystal semiconductorsubstrate either as the primary substrate of semiconductor devices or asa carrier substrate upon which semiconductor devices could be grownepitaxially. Several methods toward this end are disclosed herein, andother techniques which are modifications thereof exist. Further, incertain applications polycrystalline semiconductor membranes such aspolysilicon can be used in substitution for monocrystalline material.

It is the combination of the use of low stress free standing dielectricfilms with the appropriate processing qualities and membrane or thinfilm single crystalline (monocrystalline), polycrystalline or amorphoussemiconductor substrate formation that provides much of the advantage ofthe MDI IC fabrication process. The following methods are encompassedwithin the present disclosure:

1. Methods for the fabrication of low stress free standing (thin film)dielectric membranes that encapsulate each semiconductor device thatcomprises an IC.

2. Methods for the formation of uniform thickness semiconductor membrane(thin film) substrates for use in combination with low stress dielectricmaterials.

3. Methods for the fabrication of semiconductor devices within and on adielectric membrane that comprises a circuit membrane.

4. Methods for the formation of interconnect metallization structureswithin and on a dielectric membrane that comprises a circuit membrane.

The MDI circuit fabrication process in one embodiment starts with asemiconductor wafer substrate, and results in an IC in the form of acircuit membrane where each transistor or semiconductor device (SD) inthe IC has complete dielectric isolation from every other suchsemiconductor device in the IC. Only interconnect at the specificelectrode contact sites of the semiconductor devices provides electricalcontinuity between the semiconductor devices. The primary feature of theMDI process is complete electrical isolation of all semiconductordevices of an IC from all of the intervening semiconductor substrate onwhich or in which they were initially formed and to do so at lower costand process complexity than existing bulk IC processing methods. Otherfeatures of the MDI process are vertical electrode contact (backsideinterconnect metallization), confined lateral selective epitaxialgrowth, non-symmetric dopant profiles, and the use of a MDI circuitmembrane to serve as a conformal or projection mask for lithographyprocessing. Even if the initial substrate with which MDI processingbegins with is the most commonly used semiconductor silicon, theresulting IC need not be composed of silicon-based devices, but could beof any semiconductor device material such as GaAs, InP, HgCdTe, InSb ora combination of technologies such as silicon and GaAs grown on asilicon substrate through epitaxial means. Silicon is an inexpensive andwell understood semiconductor substrate material with superiormechanical handling properties relative to most other presentlyestablished semiconductor materials. The MDI process is not limited tostarting with a silicon substrate and the process definition of MDI isnot dependent on use of silicon; however, there are presently clearadvantages to using silicon as a starting semiconductor substrate, andthe chief embodiment disclosed herein of MDI uses a startingsemiconductor substrate material of silicon.

The benefits to fabricating an IC with the MDI process are significantover prior art methods, some of these benefits being:

1. Complete electrical isolation of semiconductor devices.

2. Vertical semiconductor device structures.

3. Lower processing costs through lower processing complexity or fewerdevice isolation processing steps.

4. Conformal mask lithography through the membrane substrate.

5. Control of depth of focus during lithography exposure due to controlof substrate thickness.

6. Application of interconnect metallization to both sides of the IC.

7. Through-membrane (substrate) interconnect metallization routing.

8. Three dimensional IC structures through the bonding of circuitmembrane IC layers.

9. Efficient conductive or radiant cooling of IC components of circuitmembrane.

10. Direct optical (laser) based communication between parallelpositioned membrane ICs.

11. Higher performance ICs.

12. Vertical semiconductor device structure formation.

13. Novel selective epitaxial device formation.

In some semiconductor technologies it is not necessary to have completeisolation between each transistor or semiconductor device, such ascertain applications using polycrystalline or amorphous TFTs (thin filmtransistors). This is not a limitation on the MDI process, becausesemiconductor device side wall isolation is an option in the MDIprocess. What is novel is that the MDI process provides general methodsby which thin films or membranes of dielectric and semiconductormaterials can be formed into a free standing IC or circuit membrane.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1a to 1j show a dielectric and semiconductor membrane substrate incross-section.

FIG. 2 shows an etched silicon substrate membrane in cross-section.

FIGS. 3a, 3b show dielectric membranes with semiconductor devices.

FIG. 4 shows an alignment mark of a circuit membrane in cross-section.

FIG. 5 shows support structures for a membrane structure isolationstructure.

FIGS. 6a to 6i show a circuit membrane Air Tunnel structure.

FIG. 7 shows stacked circuit membranes with optical input/output.

FIG. 8 shows a three dimensional circuit membrane.

FIGS. 9a to 9j show fabrication of a MOSFET in a membrane.

FIGS. 10a to 10d show fabrication of a transistor by lateral epitaxialgrowth on a membrane.

FIGS. 11a to 11f show vertical MOSFET and bipolar transistors formed ona membrane.

FIGS. 12a to 12g show transistor fabrication on a membrane usingconfined laterally doped epitaxy.

FIGS. 12h to 12j show cross-sections of selective epitaxial growth on amembrane.

FIGS. 13a to 13d show cross-sections of multi-chip modules.

FIG. 14 shows a cross-section of a membrane formed on a reusablesubstrate.

FIG. 15 shows a cross-section of the membrane of FIG. 14 with a supportframe attached.

FIGS. 16a, 16b show multi-chip modules in packages.

FIGS. 17a to 17c show soldering of bond pads of a circuit membrane to adie.

FIG. 18 shows bond pads on a die.

FIGS. 19a, 19b show bonding and de-bonding of a die to a circuitmembrane.

FIGS. 20, 21 show two sides of a circuit membrane.

FIGS. 22a to 22c show formation of a metal trace in a circuit membraneby a lift-off process.

FIGS. 23a, 23b show use of a buried etch stop layer to form a circuitmembrane having a thinner inner portion.

FIGS. 24, 25 show a source-integrated light valve for direct writelithography.

FIGS. 26, 27 are cross-sections of X-ray sources for the device of FIGS.24, 25.

FIGS. 28a to 28b show a coil for the device of FIG. 24.

FIGS. 29a to 29k show portions of a source-external radiation valve fordirect write lithography device.

FIGS. 29l to 29p show use of fixed free-standing membrane lithographymasks.

FIG. 30 shows a cross-section of a lithographic tool.

FIGS. 31a to 31c show cross-sections of a display formed on a membrane.

FIGS. 32a, 32b show bonding of two circuit membranes.

DETAILED DESCRIPTION OF THE INVENTION

The MDI process is the formation of an IC or interconnect metallizationcircuit as a free standing dielectric and/or semiconductor circuitmembrane. Each semiconductor device comprising an IC circuit membrane isa semiconductor device optionally isolated from adjoining semiconductordevices, and where each semiconductor device is formed on or in amembrane of semiconductor material typically less than 8 μm inthickness. The overall thickness of a circuit membrane is typically lessthan 50 μm and preferably less than 8 μm. The dielectric membrane iscompatible with most higher temperature IC processing techniques.

MDI Fabrication Process

Several process variations can be used to form the thin film or membraneof semiconductor material for use in the MDI process. Additional relatedmethods for forming semiconductor membranes may exist or come intoexistence and are included in the MDI technology.

Examples of some of the methods that can be used for forming siliconsingle crystal thin films are:

1. Heavily boron doped (typically greater than 10¹⁸ atoms/cm²) etch stoplayer (formed by diffusion, implant or epitaxy) with optional epitaxialSiGe (less than 20% Ge) anti-autodoping overlayer layer and optionalepitaxial layer.

2. O₂ (oxide) and N₂ (Nitride) implant etch stop barrier layer. Implantconcentrations are typically between 10 to 100 times less for formationof an etch stop barrier layer than that required to form a buried oxideor nitride dielectric isolation layer as presently done with a standardthickness silicon substrate.

3. Buried oxide etch stop barrier layer formed from a porous siliconlayer.

4. High precision double sided polished substrate and masked timedchemical etch back of back-side.

5. Electro-chemical etch stop.

6. Buried etch stop layer formation through anodic or thermal waferbonding in combination with precision substrate polishing and chemicaletching.

There are many established methods for forming thin semiconductorsubstrates or membranes. The MDI process requires that the semiconductormembrane forming process (thinning process) produce a highly uniformmembrane typically less than 2 μm thick and that the surface tension ofthe semiconductor membrane be in low tensile stress. If the membrane isnot in tensile stress, but in compressive stress, surface flatness andmembrane structural integrity will in many cases be inadequate forsubsequent device fabrication steps or the ability to form asufficiently durable free standing membrane.

The use of highly doped layers on the surface or near the surface of thesubstrate formed by diffusion, implant or epitaxial means is anestablished method for forming a barrier etch stop layer. A heavilydoped boron layer will etch 10 to 100 times slower than the rest of thesubstrate. However, if it is to form an effective uniform membranesurface, autodoping to the lower substrate and to the upper device layermust be prevented or minimized. This is accomplished in one method byepitaxially growing a SiGe layer of less than 4,000 Å (1 Å=10⁻¹⁰ m) andless than 25% Ge on either side of the barrier etch stop. The SiGelayers and the barrier etch stop layers are subsequently removed afterformation of the membrane in order to complete device dielectricisolation.

The MDI process for forming a dielectric membrane requires that thedielectric material be deposited in net surface tensile stress and thatthe tensile surface stress level be 2 to 100 times less than thefracture strength of the dielectric. Consideration is also given tomatching the coefficient of thermal expansion of the semiconductormaterial and the various dielectric materials being used in order tominimize the extrinsic net surface stress of the membrane. Thermallyformed silicon dioxide forms as a strongly compressive film and mostdeposited dielectrics currently in use form typically with compressivesurface stress. High temperature silicon dioxide and silicon nitridedielectric deposited films with tensile surface stress levels 100 timesless than their fracture strength have been demonstrated as large areafree standing membranes consistent with the requirements of the MDIprocess.

It is not a requirement of the MDI process that if a semiconductormaterial is used in the fabrication of a circuit membrane, that thesemiconductor material be capable of forming a free standing membrane.The dielectric materials that are optionally used to isolate anysemiconductor devices in the circuit membrane can provide the primarystructural means of the resulting circuit membrane as implied by thegeneral fabrication methods presented below.

The ability to form large durable temperature tolerant low tensilestress films of both semiconductor and dielectric materials ascomponents or layers of a substrate for the fabrication of integratedcircuits and interconnect structures is unique to the MDI process. Thelarge free standing semiconductor and dielectric membrane substrates ofthe MDI process provide unique structural advantages to lower the costand complexity of circuit fabrication and enhance the performance ofcircuit operation.

The MDI process can be broadly described as two methods, depending onwhich method of forming the semiconductor substrate thin film isselected. The sequence of steps of the two process methods presentedbelow may be utilized in a different order depending on processingefficiencies. Formation of polysilicon or a-Si (amorphous silicon)circuit devices on a dielectric membrane without the starting use of asemiconductor substrate is disclosed below, but is not categorized as amethod.

Method #1

1. Implant, diffuse, or epitaxially grow an etch barrier layer in overthe silicon substrate.

2. Optionally grow the desired epitaxial device layers.

3. Optionally trench isolate the semiconductor device areas.

4. Complete all desired IC processing steps including deposition of alow stress dielectric membrane.

5. Form a dielectric and semiconductor substrate membrane by selectivelyetching the back side of the substrate to a barrier layer or to acontrolled substrate residual.

6. Complete the IC processing steps on the back side of the substrateand remaining top side of the substrate, and optionally trench isolatethe semiconductor devices if not done in step 3.

Method #1 fabricates the desired semiconductor devices of an IC on astandard thickness semiconductor substrate 10 prior to the deposition ofthe low stress dielectric membrane and release of membrane structure 14by etching the back side of the substrate. (FIG. 1a shows incross-sections the substrate 10 after back side etching.) Thesesemiconductor devices will typically be trench isolated (see below)through or below the active area layer of each semiconductor device.

The semiconductor devices will typically be fabricated from vapor phaseepitaxial depositions. Well known epitaxial fabrication methods such asSEG (Selective Epitaxial Growth), ELO (Epitaxial Lateral Overgrowth),MOCVD (Metal-Organic Chemical Vapor Deposition) or MBE (Molecular BeamEpitaxy) may be employed.

In the embodiment of FIG. 1a an etch stop barrier layer 12 is implantedjust beneath the surface of the silicon substrate 10. This is shown inFIG. 1b, which is an expanded view of a portion of substrate 10 prior tothe backside etching step. Portion 11 of substrate 10 is to be removed.This barrier etch stop layer 12 may be an oxide layer produced in amanner similar to the well known SIMOX technique or an implanted siliconnitride layer; however, the thickness of the buried oxide or nitridelayer required for MDI processing is significantly less than thatrequired to achieve device dielectric isolation for a silicon substratewhich is the general application for such implant technology. (The termSIMOX (Separation by IMplanted OXygen) is a general reference for thedielectric isolation process of forming a buried layer of SiO₂ in a bulksilicon substrate by oxygen implant.) The purpose of layer 12 is toinduce minimum damage to the crystalline surface of the substrate 10which would degrade subsequent semiconductor device processing, whileproviding a uniform substrate (silicon) preferential back side etch stopjust beneath the surface of the substrate. The barrier etch stop layer12 can be made at a well controlled thickness and acts as an end pointwhen the silicon substrate 10 is selectively etched from the back side14 (see FIG. 1a) as part of the processing in forming the low stressdielectric and (optionally) semiconductor circuit membrane. The barrieretch stop 12 as shown in FIG. 1b does not extend to the edge of thesubstrate. It is not restricted from extending to the edge of thesubstrate (wafer), but it must extend beyond that portion 11 of thesubstrate 10 that will be removed during the formation of the membranestructure.

The original substrate upon which the MDI circuit membrane is formedserves as retaining frame (or ring) 18 for the MDI circuit membraneafter the backside of the substrate is etched. The backside etch of thesubstrate leaves the frame 18 of the substrate to hold the resultingcircuit membrane. The width of frame 18 is sufficient to prevent thesurface forces of the circuit membrane from causing the frame to crack.A width typically of less than 400 mils (1 cm) is sufficient. This isdone by growing thermal oxide and or depositing a film typically ofsilicon nitride (about 5,000 Å to 7,500 Å thick) on the backside 14 ofthe substrate and then etching a window or opening in dielectric layerthe size and shape of the desired circuit membrane. The substrate isthen etched through dielectric window or dielectric mask. The shape ofthe backside window opening (or dielectric mask) is typicallyrectangular, although it can vary in shape.

The barrier etch stop layer 12 is then selectively etched to leave onlythe original surface 10 layer of the silicon substrate. A barrier etchstop layer could also be formed on the surface of the silicon substrateif the layer preserves sufficient crystalline structure required forsubsequent epitaxial processing, surface stress (typically less than 10⁸dynes/cm² tensile) and there is a selective etch procedure for thesilicon substrate versus the barrier etch stop layer, and if need be,the applied barrier etch stop layer can subsequently be removed. Thebackside 14 is etched away by TMAH (tetra-methyl ammonia hydroxide) orother appropriate selective silicon etchant (see below). FIG. 1c showsformation of the semiconductor devices 24, 26, 28 in the semiconductorsubstrate after backside substrate etch to a buried etch stop layer,dielectric deposition, and selective removal of the barrier etch stop.Trenches 25, 27, 29 are cut into the substrate 10 prior to deposition ofthe low stress dielectric membrane 20, which also serves as an isolationdielectric between adjacent semiconductor devices 24, 26, 28.

An alternative technique to trench isolation as suggested by eithersteps 3 or 6 of Method #1 in order to laterally isolate transistordevices of the circuit membrane is to use the well known LOCOS (LOCalOxidation of Silicon) isolation method. LOCOS could be applied as partof the processing of step 6 (process steps 4 or 5 in Method #2 below) asshown in FIGS. 1d and 1e. FIG. 1d shows in cross-section severaltransistors 11a, 11b, 11c of a circuit membrane 20 with semiconductorlayer 20b, interconnect metallization 20c, low stress dielectricmembrane 20d, and with completed topside device processing and abackside deposited low stress silicon nitride mask 13 patterned withopenings 15a, 15b between transistors. Subsequent thermal oxidation 17a,17b of the thin silicon device substrate layer underneath the openingsin the nitride mask 13, as shown in FIG. 1e, laterally isolates thetransistors 11a, 11b, 11c and results in complete dielectric isolationof the circuit membrane transistors 11a, 11b, 11c.

The thermal oxide isolation created by the LOCOS method may change thenet tensile surface stress of the semiconductor (substrate) membranelayer. The deposition of low stress dielectric films on either side ofthe semiconductor layer prior to LOCOS processing will offset mostcompressive effects of the oxide formation. Device isolation by theLOCOS process when applied to a MDI circuit membrane is more effectivethan current bulk processing because of the shallow depth of thesemiconductor substrate layer. This also allows near optimum density ofthe circuit devices with respect to minimum device isolation separation.The easy incorporation of LOCOS into the MDI processing methods also isan indication of the general compatibility of the MDI process withexisting IC fabrication techniques.

The remaining substrate 10 (see FIG. 1f) is then (optionally) bonded atits edges to a bonding frame or ring 19 (which is typically of glass,quartz or metal and about 25 to 100 mills thick) by conventional anodic,fusion (thermal) or epoxy bonding techniques. In this case the substrateframe 18 may be removed (see FIG. 1g). The bonding frame or ring 19 isnot required for processing MDI; the original substrate 10 (which isbackside etched) performs this function initially.

An alternative to forming a barrier etch stop layer is to polish thesilicon wafer 10 on both sides to a thickness uniformity specificationsuch that when the substrate is selectively time etched from the backside, the etch can be stopped prior to reaching the dielectric membrane20 and will leave an acceptably uniform thickness for the active devicesubstrate. This method is mechanically difficult in that it requires thewafer to be exactingly polished on both sides to achieve the uniformthickness.

The dielectric membrane 20, 36 is formed as part of the interconnectmetallization dielectric and as a layer over the two-sided interconnectmetallization 35 as shown in FIGS. 3a and 3b. (FIG. 3b is an enlargedview of the portion of FIG. 3a referenced as "3b".) The thickness of thedielectric membrane 20, 36 may vary from less than 2 μm to over 15 μmper layer of interconnect metallization layer 35. All blanket dielectricmaterial covering the substrate has a low stress surface tension and ispreferably in tensile stress.

Two sided interconnect metallization 35, with low stress dielectricmaterial 36 (see FIG. 3b) used as the interconnect dielectric can beapplied, or the back side of the circuit membrane can be passivated withlow stress dielectric for a conventional one-side interconnectstructure. The circuit membrane can withstand processing temperatures inexcess of 400° C. which is required for further deposition of variouslow stress dielectric layers such as SiO₂ and Si_(x) N_(x) and toachieve reliable low resistance junctions between semiconductor deviceelectrode contacts and interconnect metallization. Depending on thespecified composition and thickness of the dielectric and substratemembrane material, higher temperature processing steps can be performedsuch as may be required by implant activation annealing or epitaxialprocessing.

The alignment of lithography tools on the back side of the substrate isperformed through the transparent dielectric material as shown in FIG.4. The semiconductor substrate material 10a near the alignment mark 40(which is nearly transparent in the visible portion of the spectrum atthicknesses less than 2 μm) can be etched away from the dielectricmembrane 20 so that the lithography alignment marks 40 are more readilyvisible.

Infrared alignment mechanisms are also available for back sidealignment; silicon is transparent at infrared wavelengths. Lithographymarks 40 for infrared alignment cannot be as small as those used in thevisible spectrum due to the longer wavelengths of the infrared,typically greater than 6,000 Å.

Method #2

1. Form a free standing low stress semiconductor substrate membrane.

2. Optionally grow the desired epitaxial device layers.

3. Complete all desired top side IC processing steps includingdeposition of a low stress dielectric membrane.

4. Optionally trench isolate the semiconductor device areas from theback side.

5. Complete IC processing steps on the back side of substrate andremaining top side of substrate.

IC processing steps used on the top side and backside of thesemiconductor membrane substrate are well known and not unique inapplication to the semiconductor substrate membrane; nearly anysemiconductor process technique can be applied.

Method #2 fabricates a thin low stress semiconductor substrate membrane20, such as the one shown in FIG. 2, prior to deposition of the lowstress dielectric membrane and fabrication of semiconductor devices.This substrate membrane as shown in FIG. 2 can be formed throughestablished selective electrochemical etching techniques or bycombination of wafer bonding, grinding and selective substrate etchingtechniques. The membrane preferably has a tensile stress ofapproximately 10⁸ dynes/cm². (This can be achieved in one manner byforming a layer with n-type dopant concentrations of 10¹⁶ to 10¹⁹atoms/cm² and applying electrochemical etch techniques.) After thesemiconductor substrate membrane 20 is fabricated (see FIG. 3a),semiconductor devices 24, 26, 28, . . . , 30 are fabricated andinterconnected on the substrate 20 with the use of low stress dielectricmaterial. The low stress dielectric membrane formed on the semiconductorsubstrate (along with interconnect metallization) becomes the onlystructural circuit membrane component after the semiconductor substrateportion of the membrane is etched or trenched into independentsemiconductor devices. An additional layer (not shown) of low stressdielectric over the interconnect metallization may be applied forpassivation and to increase the thickness of the resulting membrane 20to achieve a specific desired level of durability.

The back side etch of the semiconductor substrate 14 (see FIG. 2)permits the under side of the semiconductor devices 24, . . . , 30 to beaccessed and additional processing of the semiconductor devicesperformed. The processing options on the under side of the semiconductordevices 24, . . . , 30 are the same as those for Method #1; however,trench isolation (see FIG. 1c) of the semiconductor devices has not asyet been performed. Trench isolation is the etching of a separationspace 25, 27, 29 (typically less than 2 μm wide) between semiconductordevices 24, 26, 28 on all sides of the semiconductor devices and is anestablished IC process technique. The space or trench 25, 27, 29 is thenfilled with dielectric. Trench isolation is novel as applied here,because the technique is being applied to a semiconductor membranesupported by a dielectric membrane. If trench isolation is desired,established masking and etching techniques can be applied to formtrenches.

The uniformity of the semiconductor substrate thickness is important tothe uniformity of the operating characteristics of the semiconductordevices of an IC, and to lithography processing steps. Theabove-described embodiments use etch stop techniques when selectivelyetching the semiconductor substrate material to form the desiredsubstrate membrane thickness.

The interconnect circuit membrane can also be formed on a glass orquartz substrate 23 (typically less than 50 mils thick). In this case atensile film 25 of polysilicon (as taught by L. E. Trimble and G. E.Celler in "Evaluation of polycrystalline silicon membranes on fusedsilica" J. Vac. Sci. Technology B7(6), Nov/Dec 1989) is deposited onboth sides of the substrate 23, as shown in FIG. 1h. A MDI interconnectcircuit membrane 27 is then formed on the polysilicon membrane 25, asshown in FIG. 1i. An opening 29 in the polysilicon 25 on the backside ofthe substrate 23 is made and the back of the substrate 23 is selectivelyetched leaving a free standing tensile polysilicon membrane 27 held inthe frame 23b of the remaining substrate, as shown in FIG. 1j. Thepolysilicon 25 directly beneath the circuit membrane 27 can optionallybe removed as also shown in FIG. 1j. A metal film (not shown) or otherprotective layer is deposited over the circuit membrane prior to etchingthe backside of the substrate to protect the circuit membrane from thesubstrate etchant. Considerations for using a non-semiconductorsubstrate in the fabrication of MDI interconnect circuit membrane aresubstrate material cost or application requirements. Examples ofapplications for this approach to membrane fabrication are masks forhigh resolution conformal contact lithographic printing and MCM(Multi-Chip Module) interconnect circuits.

Low Stress Silicon Dioxide and Silicon Nitride Deposition Recipes

Low stress dielectric membranes have been manufactured consistent withMDI requirements for low stress high temperature dielectric films. Thesemembranes were produced on Novellus Systems, Inc. (San Jose, Calif.)Concept One dielectric deposition equipment, but are not limited to suchequipment. Low stress is defined relative to the silicon dioxide andsilicon nitride deposition made with the Novellus equipment as beingless than 8×10⁸ dynes/cm² (preferably 1×10⁷ dynes/cm²) in tension.Acceptable surface stress levels of different dielectrics made onvarious equipment may vary widely.

The following are two typical recipes used to produce the dielectricmembranes of silicon dioxide or silicon nitride required by the MDIprocess. Variations of these recipes can be used to emphasize specificcharacteristics of the deposited dielectric membrane; these recipes arenot the only recipes to achieve the requirements of the MDI process onthe Novellus equipment and are not limitations on the MDI process. Smallvariations in the parameters of the recipes can produce changes in thematerial structure, etch rate, refractive index, surface stress, orother characteristics of the deposited dielectric material.

    ______________________________________                Recipe #1   Recipe #2                Silicon Dioxide                            Silicon Nitride    ______________________________________    Temperature    400° C.                                400° C.    Pressure        1.8 Torr     2.3 Torr    HF RF Power    640 watts    220 watts    LF RF Power,    100 ohms       160 watts    180 watts    SiH.sub.4      260 sccm      0.23 slm    NH.sub.3      --             2.00 slm    N.sub.2       1150 sccm      0.60 slm    N.sub.2 O     6000 sccm     --    ______________________________________

Structurally Enhanced Low Stress Dielectric Circuit Membranes

FIG. 5 shows a structurally enhanced MDI circuit membrane structure.Structural enhancement of the MDI circuit membrane may prove necessaryfor various applications where stress is applied to the membrane as partof normal operation, such as in pressure sensing or in making contacttest measurements as in IC wafer sort testing. A portion 44 of thedielectric membrane is deposited over the SD layer 24a with a thicknessgreater than 1 μm, and typically 10 to 25 μm. This thicker depositedlayer 44 of the dielectric membrane is patterned with a mask and dryetched to achieve a honeycomb-like pattern of recesses 46a, 46b, 46c.The depth of these recesses 46a, 46b, 46c is approximately 75% of thethickness of the low stress dielectric 44 in which they are etched.Optional electrical contact 47 is provided and shown as an example of acircuit electrode. The dimensions of the opening of the recesses 46a,46b, 46c are typically two or three times the depth dimension. Adeposition of 1,000 Å of low stress CVD silicon nitride may optionallybe applied to form a passivating seal.

MDI Air Tunnel Interconnect Structure

The impedance of metal conductors (traces) that make up the interconnectmetallization between semiconductor devices and passive circuit elementssuch as capacitors or resistors must be given careful designconsideration at operational frequencies in excess of 100 MHz. Thedielectric constant of a dielectric (insulating) material is a primarydetermining factor when consideration is given to the use of thematerial. Polyimide materials conventionally used in the construction ofinterconnect structures have dielectric constants typically rangingbetween 2 and 3.5. The dielectric constants of CVD silicon dioxide andsilicon nitride typically range upward from 3.5. The ideal dielectric isvacuum, gaseous or air with a dielectric constant of approximatelyunity. The fabrication of an interconnect structure that isolates themajority of a conductor's surface area with vacuum or gaseous dielectricresults in a near optimum condition for high speed operation of thatstructure. Trace or conductor structures called "Air Bridges" arefabricated on the surface of an IC contacting the surface of the IC onlyperiodically. This periodic contact provides mechanical support and orelectrical contact with a low net dielectric constant of isolation. AirBridges are conventionally used in the fabrication of microwavecircuits; such circuits have operational frequencies in the GHz range.

FIGS. 6a, 6b and 6c show cross-sectional views of a conductor structure50 internal to dielectric layer 52, 52a, but which has a gaseous primarysurface dielectric contact with only periodic mechanical contact 50-a,50-b, 50-c, like the conventional Air Bridge. The conductor 50 is heldsuspended in the gaseous dielectric without contacting the surroundingor enclosing solid material structures. FIG. 6d is a top view of aportion of a conductor or trace showing column supports (and/or viacontacts) 50-a, 50-b, 50-c and dielectric support columns 52-a, 52-b.This interconnect structure is herein called an "Air Tunnel", and itsmethod of fabrication is a direct extension of the methods used in theabove-described fabrication of the MDI circuit membranes (dielectric andsemiconductor membranes). Also shown in FIGS. 6a, 6b and 6c aredielectric membrane 56, semiconductor membrane 58, dielectric supportcolumn 52a, ground plane metallization 60, cavity 54 formed by etchremoval of a-Si (amorphous silicon), and opening 62 for etch removal ofall the a-Si. FIG. 6b is an end-on view of an alternate structure toFIG. 6a. FIG. 6b shows dielectric plug contact 68 to support (suspend)conductor 50. FIG. 6c shows an extension of the structure of FIG. 6awith a second layer of dielectric 70 and traces 72.

The Air Tunnel structure can be fabricated on any semiconductorsubstrate in addition to an MDI circuit membrane. The structuraldielectric materials used are selected from the same group of low stressdielectrics used in the fabrication of the MDI circuit membranesdiscussed above. The fabrication method for the Air Tunnel provided herecan also be extended to the gaseous dielectric isolation ofsemiconductor devices and passive circuit elements in a circuitmembrane.

The Air Tunnel structure in one embodiment is fabricated with CVDprocessing techniques; alternatively, ECR (Electron-Cyclotron-Resonance)plasma CVD processing may soon provide an alternative deposition method.The gaseous dielectric separation of a conductor or a semiconductordevice is accomplished by forming a sacrificial CVD film of a-Si,polysilicon or alternate material (typically dielectric material) thatcan be deposited by CVD means and selectively etched versus the otherMDI circuit membrane material layers. Anticipating the use of Air Tunnelinterconnect structures in the fabrication of a circuit membrane,semiconductor devices are isolated (passivated) by trench isolation ofeach device, and depositing a thin layer of oxide or nitride (typicallyless than 2,000 Å thick) over exposed device surfaces, and thendepositing a film of a-Si. The thickness of the a-Si film and the widthof the isolation trench are chosen such that the trench is plugged orfilled evenly leaving the surface over the plug relatively planar. Thisplug technique is facilitated by CVD process technology which depositsfilms conformally on all interacting surfaces. Subsequent Air Tunnelinterconnect structures are completed and the a-Si layers are removed bya silicon selective etchant; the etchant accesses the a-Si through theetch-vias as explained below.

The fabrication method of Air Tunnel interconnection is independent ofthe underlying substrate. The underlying substrate could be a MDIcircuit membrane as shown in FIG. 6d (fabrication of which is disclosedabove), a conventional IC substrate of semiconductor devices, a MCMcircuit substrate, MDI tester surface membrane, etc.

The Air Tunnel fabrication process begins (see FIG. 6f) with thedeposition of an a-Si film 76 onto a substrate 56 with electrodes to beinterconnected. The a-Si film 76 is patterned with trenches and contactvias. A CVD processed metal film 50, typically tungsten (W), isdeposited over the a-Si. The dimensions of the a-Si trenches and vias inthe a-Si layer are consistent with forming planar plugs. The trenches inthe a-Si 76, once plugged with the metal film 50, become the supportcolumns 50-a, 50-b, 50-c of the conductor, see FIGS. 6a and 6d. Theconductors deposited in the vias provide mechanical (column) support andelectrical contact. The metal film 50 is patterned and a second CVDprocessed a-Si film 78 deposited over the metal film 50; a thin layer(not shown in FIG. 6a) of highly conductive metal like Au or Cu may besputter deposited over the CVD metal (prior to the patterning of themetal film 50) to enhance conductivity of the conductor. The a-Si film78 is patterned as shown in FIG. 6f, removing the a-Si film 78 from overthe metal conductor 50 and forming a trench 80 along the edge of theconductor 50. This trench is plugged by a 3rd CVD layer 82 of a-Si, asshown in FIG. 6g. The patterning of the second a-Si layer 78 over themetal conductor 50 does not require a critical alignment; an alignmenttolerance of +50% of the thickness of the second a-Si layer 78 issufficient.

The resulting three films 76, 78, 82 of a-Si are relatively planar withminor surface features along the edges where conductors 50 are directlybeneath. These surface features can be reduced by shallow thermaloxidation of the a-Si surface and subsequent stripping of the oxide (notshown).

An alternative method that can be used to planarize the a-Si and metalfilms is to spin coat a thick polymer over substrate 75, low stressdielectric 77, first a-Si layer 79, and second a-Si layer 81 after theapplication of the second a-Si film 81. (This a-Si film 81 is depositedin thickness equal to the combination of the former second and thirda-Si films.) The polymer 85 used is selected to have a RIE rate nearlyidentical to the that of the a-Si film. The polymer 85 is completelyremoved and the etch of the a-Si is continued as desired, see FIGS. 6hand 6i.

The a-Si film stack is then patterned to form trenches that will becomethe supporting columns 50-a, 50-b, 50-c of an over layer of low stressdielectric. These support columns are typically placed in anoncontinuous manner along conductors 50 (as shown in FIG. 6d), andperiodically placed across open areas 51 where there are not conductors.The periodic and overlapping placement of the support columns(etch-vias) increase the ability of a-Si selective etchant to remove alldeposited a-Si material. The thin metal film 60 (see FIG. 6a) oftypically less than 5,000 Å thickness may optionally be deposited overthe patterned a-Si prior to the deposition of the low stress dielectricfilm to act as a ground plane and EM (Electro-Magnetic) shield for theconductor 50. (This metal film remains in contact with the overlying lowstress dielectric film after the a-Si has been removed.) A low stressdielectric film 52, 52a is deposited of typically 1-2 μm thickness overthe patterned a-Si films. This low stress dielectric layer 52, 52a ispatterned with trench openings 62 to the a-Si films. These trenchopenings are called etch-vias and provide access for a a-Si selectiveetchant like TMAH or ethylene diamine to remove all of the internallayers of a-Si films from each layer of interconnect without regard tothe number of layers.

The etch-vias 62 in the dielectric are placed near dielectric supportcolumns as shown in FIGS. 6c and 6d. The etch-vias 62 are placed onevery interconnect layer and with sufficient frequency per layer toallow etchant to reach all lower layers of the Air Tunnel interconnectstructure. After the etch-vias 62 are formed an additional interconnectlayer 72, 80 (see FIG. 6c) can be formed by repeating the sequence ofprocess steps as outlined below. When the Air Tunnel interconnectstructure is used on a semiconductor membrane to form a circuit membrane(MDI process), it should be clear that the application of the Air Tunnelto both sides of the semiconductor membrane is a natural extension andnot a special case application requiring a different processingsequence.

The etch-vias 62 serve as access ports for gaseous material (such as N₂or air) which allow the effective circuit dielectrics to enter thecircuit membrane. The etch-vias 62 also serve to maintain equalizedpressure between the internal interconnect structure and the externalenvironment of the surface of the circuit.

The following sequence of processing steps for fabricating an Air Tunnelassumes that the substrate being processed has an existing layer of lowstress dielectric film on which to start:

1. Pattern dielectric for metal contact plug vias and etch-vias.

2. Deposit a film of a-Si; the thickness of this film will determine thelower separation distance of the tunnel conductor from the underlyinglow stress dielectric.

3. Pattern a-Si film for conductor support columns (vias to underlyingdielectric).

4. Deposit metal conductor film.

5a. Pattern metal conductor film.

5b. Optionally deposit high conductivity metal film before patterningconductive film.

6a. Deposit a-Si film approximately the thickness of the metal film.

7a. Pattern a-Si film to expose conductor surfaces and trench etch alongsides conductors.

8a. Deposit a-Si film to planarize the surface and separate theconductor from the overlying dielectric film.

9. Pattern a-Si film for dielectric support columns (vias todielectric).

10. Optionally deposit ground/shield metal film.

11. Deposit low stress dielectric film.

12. Pattern dielectric with etch-vias or repeat from first step above.

Steps 6 through 8 from above can be replaced with the followingalternative steps for forming a planarized conductor layer:

6b. Deposit a-Si film with a thickness equal to the thickness of theconductor and the desired separation over the conductor.

7b. Deposit planarizing polymer with a etch rate that is very similar tothat of the a-Si film, as shown by FIG. 6h.

8b. RIE until all polymer material has been removed, as shown by FIG.6i.

The principle attributes of the Air Tunnel are:

1. Low average dielectric constant (approaching unity).

2. Self planarization fabrication method.

3. Integral ground or EM shielding plane.

4. Application to one or both sides of a circuit membrane.

5. Application to MDI circuit membranes or standard wafer substrates.

A MOSFET device connected by Air Tunnels is shown in cross-section as anexample in FIG. 6e. The transistor of FIG. 6e is formed from asemiconductor membrane and held suspended by the metal contacts to itselectrodes with a gaseous dielectric separation on all remaining devicesurfaces. Shown in FIG. 6e are the active portion 84 (MOSFET) and theunused portion 86 of the semiconductor membrane, a thin nitrideisolation film 88, mechanical conductor support 94, conductor trace 92,and additional air tunnel interconnect structures 92, 94, 96.

MDI Circuit Membrane Advantages

The fabrication of circuit membranes provides the capability tofabricate and use integrated circuits in novel ways such as:

1. Back side interconnect metallization, backside SD electrode contact,and through-the-dielectric membrane interconnect metallization signalrouting. (See FIG. 3b).

2. The use of optical communication (see FIG. 7) in a vertically arrayedstack of MDI circuit membranes 150a, 150b, 150c with optical transmitand receive semiconductor devices on either side of the circuitmembranes. Busing of data through a stack of several MDI circuitmembranes 150a, 150b, 150c from any point on the surface of an IC ratherthan forming connections at the edge of ICs (which is currently thepractice) simplifies the structure of the circuit. An externalcommunication optical transceiver 152 is provided. MDI circuit membrane150b includes a transparent window 154 through which opticaltransmissions from transmitters (laser diode arrays) 156 pass to opticalreceiver SDS 158. The stack of MDI circuit membranes is held together bysupport 160. The ability to use optical means rather than metalconductors to transmit information between ICs shortens the length ofthe communication path, increases the speed and bandwidth of thecommunication path, and lowers the power consumed compared to presentmetal connection methods.

The optical receiver of the MDI circuit membrane is typically less than5,000 Å in thickness. A receiver of this thickness will absorb only aportion of the optical signal fluence striking it, the remainder of thefluence will pass through and out the opposite side of the receiver.This permits a second or third receiver to be positioned in the path ofthe optical transmitter to receive the same signal. The limitingconditions that determine the number of receivers that can be associatedwith one transmitter are the output fluence of the optical transmitterand the thickness of the transceiver substrate. This ability tooptically transmit to several receivers simultaneously has the benefitsof reducing circuit complexity (versus a transceiver structure at everycircuit membrane interface) and performance (no transceiver (repeater)propagation delay).

3. The vertical bonding of two or more circuit membranes to form a threedimensional circuit structure, as shown in FIG. 8. Interconnectionbetween the circuit membranes 160a, 160b, 160c including SDs 162, 164,166 is by compression bonding of circuit membrane surface electrodes168a, 168b, 168c, 168d (pads). Bonding 170 between MDI circuit membranesis achieved by aligning bond pads 168c, 168d (typically between 4 μm and25 μm in diameter) on the surface of two MDI circuit membranes 160b,160c and using a mechanical or gas pressure source to press the bondpads 168c, 168d together. If the pads 168c, 168d are solder, they can beheated to the melting point of the solder (typically less than 350° C.)causing the pads to weld to each other. If the pads are indium, tin, oralloys of such metals, a bond 170 will form between the metal pads 168c,168d with the application of approximately 100 p.s.i. pressure and anapplication temperature between 50° C. and 400° C., depending on themetal or alloy selected.

4. MDI Nanometer width gate MOSFET device fabrication methods. Theability to fabricate a MOSFET device with a gate region length of lessthan 0.5 μm (500 nm) presently requires lithograph tools with greaterresolution capability than the current high volume optical steppertools. Several methods are presented here for the fabrication of MOSFETwith gate lengths of less than 500 nm and capable of gate lengths lessthan 25 nm. These methods take advantage of the MDI process to formtransistor gate regions of less than 500 nm (0.5 μm) withoutlithographic means are described hereinafter.

5. Sensor diaphragms formed with integrated circuits fabricated onconventional rigid substrates.

6. Fabrication of electrically isolated semiconductor devices on bothsides of the semiconductor layer of a circuit membrane.

V-Groove Transistor Region Gate Formation Method

FIGS. 9a to 9e show a sequence of steps for forming a p-channel orn-channel (npn) transistor with an opposed gate electrode and a gatewidth of less than 25 nm (250 Å). The process steps (see FIG. 9a) assumea starting substrate of an MDI circuit membrane with approximately 0.5to 1.5 μm wide isolated metal gate electrode 174 fabricated on thebackside (opposed) of the transistor. This MDI circuit membrane consistsof a lightly doped <100> crystalline silicon membrane layer 176 ofthickness typically less than 2 μm and a 1 to 2 μm layer of low tressdielectric 178. The sequence of process steps for one embodiment is:

1. Form lightly doped p and n wells. FIG. 9b shows one such well 180.This is done by depositing a nitride layer 182 over the silicon layer176 of the MDI circuit membrane, patterning the well 180 and implantingthe desired dopant.

2. Deposit dielectric 184 (see FIG. 9c) and pattern an opening 0.75 to1.25 μm wide over the opposed gate area, but a known width.

3. Anisotropically etch along the <111> crystalline planes of thesilicon to form a V-groove 186 of known depth and side angle of 54.7degrees. This is done with a time rated etch.

4. Deposit by CVD equipment a metal layer 190 such as tungsten of 0.5 to1.5 μm, but of known thickness, just closing the V-groove 186.

5. Etch back the metal layer 190 until a known thickness of metal plugremains in the V-groove 186. The width of the resulting transistor gatelength is determined by the dimension of the remaining metal in theV-groove.

6. Strip the dielectric mask 184 in FIG. 9c.

7. Etch the silicon 180 surface to just expose the metal plug 190opposed to the gate 174.

8. Deposit a dielectric layer and pattern for heavy implant doping 192of transistor source and drain regions. This is done with n and p dopingseparately leaving a lightly doped gate channel region 194.

9. Strip the dielectric mask in FIG. 9e and etch the silicon 180 surfaceuntil it is below the level of the metal plug 190, causing its removal.(This step is optional, the metal plug can be selectively etch removed.)

10. Form trench 196 to isolate transistors to under the layer of lowstress dielectric 178.

11. Anneal the active transistor implants 192.

12. Form source and drain contacts 198.

13. Anneal the source and drain contacts 198.

It should be clear that the opposed gate electrode which is much widerthat the actual length of the gate width could have been fabricated ontop of the transistor with the source and drain contacts, or after thegate region of the transistor was formed.

The reliable fabrication of the gate region 194 at a known length isdependent on etch rate control. Present etch rate techniques support thecapability to form a gate region 194 length of less than 25 nm. A CVDdielectric could also be used as the gate implant mask instead of ametal plug 90. This gate fabrication method can also be done in bulksilicon and is not limited to the MDI process. It is important to notethat the sub-micron resolution of the gate region was achieved withoutlithographic means capable of sub-micron resolution.

FIGS. 9f, g, h, i & j show the process sequence for substrate-imagednonlithographic MOS transistor gate region or Bipolar emitter regionformation. This process takes advantage of the novel ability of the MDIstructure to accommodate process steps on either side of the membranesubstrate. This process can form a minimum feature size of less than 100nm and does so by the use of anisotropic etching epitaxial processes.

FIG. 9f shows a MDI substrate of semiconductor 189 and low stressdielectric 191 formed by one of the methods presented above. Thedielectric layer 191 on one side has been patterned and thesemiconductor layer 189 anisotropically etched through to the dielectriclayer 193 on the opposite side. The thickness of the semiconductor layer189 is selected to be less than 2 μm, and the dielectric layers 191, 193are less than 1 μm thick each, though these dimensions can be scaled toachieve any transistor dimensions. The size of the patterned opening 195in the dielectric is chosen such that the etched opening 197 onto theunderlying surface of the dielectric is a desired width, less than 100nm. Widths of any dimension can be created, however, widths of less than100 nm are the primary objective of this process which cannot beachieved by optical means. The patterned dielectric 191 has a1,000-2,000 Å layer 199 of low stress silicon nitride to provide highselectivity versus the opposed dielectric layer during a subsequent RIEprocess step.

FIG. 9g shows an opening 197 formed through the dielectric layer 193opposite the patterned layer 191. The anisotropically etchedsemiconductor layer 189 with its opening 197 onto the under side of theopposite side dielectric was used as a substrate-imaging mask, and thedielectric layer 193 was etched by RIE (dry) processing. Thissubstrate-imaged opening 197 can be less than 100 nm in diameter andwill become the gate region of a MOS transistor or the emitter region ofa bipolar transistor. FIG. 9h shows the substrate-imaged region 197closed by selective epitaxial growth 201; the epitaxial growth 201 canbe formed with a graded dopant structure to lessen short channeleffects. FIG. 9i shows the formation of a gate oxide 203 and electrode205 after a planarization step of the self-imaged opening 197, CVD orthermal formed gate oxide and formation of gate electrode. It isimportant to note that the dielectric layer 193 in which the gateelectrode 205 is formed is self-aligned to the gate region.

FIG. 9j shows the finished MOS transistor. The side opposite the gateelectrode 205 is stripped of dielectric, planarized and etched to athickness of less than 2,500 Å. It was then trenched 207 forsource/drain isolation, a layer of low stress dielectric 209 depositedand source/drain electrodes 211, 213 formed.

It should be clear to some skilled in the art that a bipolar transistorcan be formed with somewhat similar fabrication steps.

MDI Lateral Epitaxial Grown Transistor Gate Region Fabrication

FIGS. 10a, b, c, & d show a method for forming a MOSFET transistor withtransistor gate region lengths of sub-micron dimensions, without therequirement for lithography tools with sub-micron CD resolutioncapability. This method is an extension of the above-described MDIprocess technology.

In FIG. 10a, the starting substrate structure is a combination siliconand dielectric 204 film forming a membrane 202. This membrane 202 isformed by one of the methods disclosed above. Subsequently, membrane 202is patterned for the doping of the n+ and p+ transistor channels. FIG.10a shows a cross-section of a MOS transistor channel (source 210, gate208 and drain 206 regions) with a nitride mask 212 through which ananisotropically formed gate 208 trench is etched to the underlyingdielectric membrane 204 in the location where the gate region of thetransistor will be constructed. The exposed silicon sidewall 214 may bepolished by a thin thermal oxidation and etch stripping or chemicaltechniques. FIG. 10b shows a top view of the structure of FIG. 10a, withthe transistor channel 210, 208, 206 isolated from surroundingtransistor devices by dielectrically filled trenches 216. These trenches216 are fabricated prior to etching the gate region trench 208. The gateregion trench 208 is then filled (see FIG. 10c) from both siliconsidewalls by lateral selective epitaxial growth of silicon 220 and withthe same or similar doping concentration as the adjacent regions 210,206; source and drain implant doping can also be used as a doping meansprior to epitaxially growing the gate region. (In the case of CMOSdevices, the n+ and p+ transistor channels are processed in two separatesequences of processing steps.)

The epitaxial growth process in the gate region trench 208 is stoppedwhen the remaining separation distance "d" between the approachingsilicon sidewalls 220 is equal to the desired gate region length. Thedoping concentration is changed for forming the actual gate region 222,and the epitaxial growth is continued until the gate region trench isclosed, as shown in FIGS. 10c and 10d.

The width of the initial gate region trench 208 and the epitaxial growthrate can be controlled with an error of less than 5 nm. This allows thelength d of the gate region to be determined by the precision with whichthe gate region trench 208 can be formed, the deposition rate control ofthe lateral epitaxial growth 220 and the capability to create thedesired doping transitions. The result of this approach means that agate region length d of 25 nm or less can be formed. This is far beyondthe current optical lithographic methods which are presently expected toreach production capability of 120 nm by the end of the decade.

FIG. 10d shows the completion of the transistor with source and drainelectrodes 226, 224, and an opposed gate electrode 230. The surface ofthe epitaxially grown gate region 222 is not smooth, but the backside ofthe gate region 222 is smooth, due to its formation against theunderlying low stress dielectric membrane 204. The gate electrode 230 ismore easily and reliably formed by patterning the underlying dielectricmembrane 204 from its backside, thereby forming the opposed gateelectrode 230. The backside of the gate region 222 is etched free ofdielectric with an opening significantly larger than the length d of theepitaxially grown gate region 222; portions of the source 210 and drain206 regions are also exposed with the gate region due to lithographictool limitations. Gate oxide isolation 232 is thermally grown ordeposited to the required thickness, and a metal or silicide electrodeis formed 230.

The overlap of the gate electrode 230 over the source 210 and drain 206regions of the transistor channel results undesirably in performancelimiting capacitance. The reduction of the capacitance from theoverlapping gate electrode structure can be achieved by either epitaxialgraded doping of the source and drain regions at the interface to thegate region growth over the gate region, to form an extended gateregion, or by offset of the gate electrode so that the source side edgeof the electrode aligns approximately with the source/gate interface ofthe transistor channel. These methods are disclosed below. Theabove-described MOSFET transistor structure can be converted to abipolar transistor structure as shown in FIG. 12g, also described below.

The MDI circuit membrane structure provides the capability to positionelectrode contacts of semiconductor devices at their backside (oropposed side). This provides significant savings in reduced processingsteps to achieve device isolation and new transistor structures.

FIG. 11a shows a MDI circuit membrane (dielectric 240 and semiconductor242 films) in cross-section on which additional semiconductor layerswere formed epitaxially to achieve either an npn or pnp transistorstructure 244. The epitaxial layer 244-a that corresponds to the gateregion can be formed with a thickness of less than 25 nm. FIG. 11b showsthe dielectric 240 and semiconductor 247, 244, 245 membrane aftersubsequent well known semiconductor processing steps which form atransistor channel 244 isolated on the dielectric membrane 240 withsource 246, drain 248 and gate electrodes 250-a, 250-b attached. Alsoshown are source region 245 and drain region 247. The gate oxideisolation 251 is thermally grown or deposited, and gate contacts 250-a,250-b can be formed on one or all sides of the transistor channel.Alternatively, a separate contact 250-b can be formed opposite the gateelectrode 250-a, and used as a transistor bias contact. The opposeddrain contact 248 is formed by etching a via in the dielectric membrane240 underlying the transistor drain region 247.

FIG. 11c shows a bipolar transistor formed by a well known method but ona low stress dielectric membrane 254, and without the requirement toform a lateral conductive path (typically referred to as a buriedlayer). Formed in semiconductor film 256 are emitter 258, base 260, andcollector 262 regions. The collector contact 264 is instead formeddirectly under the emitter 258. It should also be apparent that thereare non-silicon semiconductor transistor structures that can berestructured with a backside contact as depicted in FIG. 11c, such as aGaAs Heterojunction Bipolar Transistor (HBT) or Ballistic Transistor.

The extension of the gate electrode 250-a beyond the gate region in thefabrication of the MOSFET transistor structure of FIG. 11b increases theparasitic capacitance of the transistor, and therefore, adversely affectits performance. This capacitance can be reduced by the separate orcombined use of several methods referred to as Graded Gate Doping (GGD),Epitaxially Extended Gate (EEG) and Gate Electrode Offset (GEO).

The Graded Gate Doping is shown in FIG. 11b. This method varies thedoping levels of the source 245 and drain regions 247 at theirrespective interfaces to the gate region. This directly decreases theeffective dielectric constant of the semiconductor under the dielectricisolation 251 separating the gate electrode from the source 245 anddrain 247 regions it overlaps.

The Epitaxially Extended Gate 244-b (see FIG. 11d) is formed byselective epitaxial growth of the gate region 244-b over the surface ofthe transistor upon which the gate oxide 251 and gate electrode 250-awill be fabricated. This method, more directly than the GGD method ofFIG. 11b, lowers the capacitance of the semiconductor region directlyunder the gate electrode 250-a where it overlaps the source 245 anddrain 247 regions. The transistor capacitive charging effect contributedby the gate electrode 250-a from source 245 and drain region 247 overlapis decreased by these methods, because of the dopant type and dopantconcentration relative to the polarity of the applied potential on thegate electrode 250-a. The thickness of the epitaxial layer of the EEGmethod is determined based upon the desired operating characteristics ofthe transistor, but typically is less than the dimension of the gateregion length.

The extended gate region 244-b of FIG. 11d formed by the EpitaxialExtended Gate method can also be a graded doping region. If the extendedgate region 244-b is graded, the dopant concentration decreases goingaway form the gate region (or towards the gate electrode 250-a).

The Gate Electrode Offset (GEO) structure is shown in FIG. 11f. Shownare source region 269, gate region 271, drain region 273, semiconductorlayer 275, low stress dielectric layer 277, gate oxide 279, and offsetgate electrode 281. The drain side edge of the gate electrode 281 isaligned to the drain/gate 273-271 interface of the transistor channel.The alignment could alternately be made to the source/gate 269-271interface, and would be a preferred structure under certain devicedesign requirements. The accuracy of the placement of the electrode 281edge is limited by the capability of the lithography tool used. A gapbetween the drain region 273 and gate electrode 281 may result leaving aportion of the gate region 271 not covered by the gate electrode 281 dueto lithography alignment deficiencies. This can be corrected byextending the drain region 273 through implant doping of the exposedgate region. The size of this gap during manufacturing can reliably beexpected to be no greater than the magnitude of the alignmentregistration error of the lithography tool being used.

The transistor structure of FIG. 11e used as a bipolar transistor is ofinterest in order to achieve well controlled fabrication of thin baseregions. The contact 270 to the base region 272 of a bipolar transistorin FIG. 11e is achieved by epitaxially extending 272-a the base region.This method like the EEG method is the selective epitaxial growth of anexposed region of the transistor that includes the base region 272.Other elements of FIG. 11e are collector region 274, collector contact275, dielectric membrane 276, emitter region 277, and emitter contact278.

MDI Transistor Fabrication Based on Confined Laterally Doped Epitaxy

Selective growth of high quality epitaxial films in a confined volumewas reported in a paper ("Confined Lateral Selective Epitaxial Growth ofSilicon for Device Fabrication," Peter J. Schubert, Gerold W. Neudeck,IEEE Electron Device Letters, Vol. 11, No. 5, May 1990, page 181-183).This "CLSEG" (Confined Lateral Selective Epitaxial Growth) method wasdeveloped to achieve dielectric isolation of crystalline silicon.

FIGS. 12a, b and c show the use of a MDI circuit membrane to produce aconfinement cavity for the epitaxial growth of crystalline silicon. Theoriginal objective of CLSEG, to fabricate a dielectric isolatedsemiconductor substrate, is not an objective here because the MDIprocess is itself a dielectric isolation process. What is taught here isthe application of the capability for growing epitaxial films in aconfinement cavity within the context of the MDI process, and aprocessing method for using confined lateral epitaxial growth tofabricate MOSFET transistors with arbitrarily narrow gate region lengthswithout the use of lithographic means.

MDI confined lateral doped epitaxial closely controls transistorgeometry in all three dimensions: height (as deposited semiconductorfilm thickness), width (placement of isolation trenches) and source,gate, drain regions (epitaxial deposition thickness). This makes thedevice operational characteristics more predictable with lowermanufacturing complexity.

The MDI Confined Lateral Epitaxy method begins with a membrane of twofilms, one film 280 of semiconductor (silicon) and one film 282 of lowstress dielectric. This membrane 280, 282 is made by one of the methodsdisclosed above. Dielectric filled isolation trenches 284 (see FIG. 12b,a top view of the structure of FIG. 12a) are fabricated to form whatwill become the sides of the transistor channel in the semiconductorfilm. Then a low stress dielectric layer 290 typically of 1 μm or lessthickness is deposited, and a window 286 is anisotropically etchedthrough the semiconductor film 280 stopping on the dielectric layer 282and positioned between the isolation trenches 284, as shown in FIGS. 12aand 12b. In the preferred embodiment the orientation of the window 286is approximately aligned with the crystallographic orientation of thesemiconductor film 280 in order to produce a seed crystal walls 292-a,292-b of FIG. 12c that is at a right angle to the isolation trench walls284, 290, 282 in the confinement cavity, as explained below. Thetransistor depth (thickness) is well controlled by the thickness of thesemiconductor membrane 280 which in turn can be precisely determined byepitaxial and/or etching means.

The exposed semiconductor film 280 is then anisotropically etched backin two directions under the dielectric over layer 290 and along theisolation trenches 284 as shown in FIG. 12c. This forms a very smoothwalled confinement cavity 292 with a smooth seed crystal 292-a, 292-b atthe end of the cavity 292. The etch is stopped at a lateral depthconsistent with the design requirement for the number of transistors tobe formed in the confinement cavity. The depth "c" of the confinementcavity 292 may vary from about 2 μm to over 15 μm.

A smooth contamination free seed crystal wall 292-a, 292-b is requiredfor the subsequent epitaxial growth. The crystal seed wall 292-a, 292-bwill form at an angle dependent upon the crystallographic orientation ofthe semiconductor film. A <100> silicon film will form a seed wall angleof approximately 54.74° as measured from the horizonal, and a <110>silicon film will form a seed wall 292-a, 292-b with an angle 900 (asshown in FIG. 12c). GaAs and InP have similar crystallographicproperties and can be used as the semiconductor film instead of silicon.

The selective epitaxial growth of crystalline semiconductor proceedsfrom the seed crystal wall 292-a, 292-b. The npn or pnp regions ofsilicon MOSFET transistors are epitaxially grown (see FIG. 12d) with insitu doping to whatever design thicknesses desired. This allows thelength of the base or gate region of the transistor to be determined bythe epitaxial growth process instead of a lithographic process aspresently done. This also allows the length of the base or gate regionof the transistor to be arbitrarily narrow, which cannot be done withcurrent lithography methods. FIG. 12d shows the confinement cavity 292filled with a CMOS transistor pair of npn and pnp structures 294, 296.Each region of the transistor can be uniquely tailored to length anddoping concentration level, in addition, because the transistor channelis formed by epitaxial means, doping of the transistor 294, 296 regions,can be extended to graded segments as discussed above. Further,non-symmetric doping of source and drain regions can easily beimplemented by time based selection of dopant during epitaxial growth;this is not easily accomplished in bulk wafer processing due lithographylimitations in resolving the gate region.

FIG. 12d shows two pairs 294, 296 of CMOS transistors formed as mirrorimages of each other in two confinement cavities; this was a designchoice because a single confinement cavity could have been formed byadditional trench dielectric isolation of the initial confinement windowopening. Depending on the method of epitaxial growth, gate regionlengths "g" of less than 25 nm can be fabricated. In contrast, prior artMOSFET transistor gate regions are predominately formed by opticallithographic means and are presently limited to minimum gate regionlengths of approximately 0.5 μm (500 nm).

FIG. 12e shows the CMOS transistor pairs 294,296 with electrode contacts294-a, 294-b, 294-c, 296-a, 296-b, 296-c. The isolated gate electrodes300, 302, 304, 306 each span the narrower gate region length of thetransistor. The gate electrode is formed by standard lithographicprocesses, and may be many times of greater length than the gate regionlength of the transistor. For example, a lithographically defined gateelectrode of 0.5 μm and a transistor gate region length of 25 nm arepossible. Gate electrode capacitance attributed to the gate electrodeoverlap of the source or drain regions can be reduced as described abovewith the GGD, EEG, and GEO methods. FIG. 12f shows embodiments of all ofthese methods; the electrode 312 opposed to the gate electrode 308 ispresented optionally as a substrate bias. Also shown in FIG. 12f aresource region 294-1, optionally graded doped gate region 294-2,epitaxially extended gate regions 294-4, offset gate electrode 308, gateisolation 310, and optionally epitaxially extended substrate 312-a.

The MDI Confined Lateral Epitaxy method is not limited to MOSFETtransistors or silicon as a semiconductor material. The MDI ConfinedLateral Epitaxy method can be used to fabricate silicon bipolartransistors, or applied to other semiconductors such GaAs or InP whichcan be epitaxially formed. An example of a bipolar transistor is shownin FIG. 12g when emitter region 314, base region 316, epi-extended baseand electrode 318, collector 315, and optionally epi-extended base andelectrode 319.

The semiconductor layer of a MDI circuit membrane provides variousmethods to pack circuit devices densely, mix semiconductor types orsimplify the fabrication steps. FIGS. 12h, i & j show examples of suchmethods. FIG. 12h shows a silicon membrane 289 upon which an epitaxialdielectric 291 and a subsequent layer of epitaxial semiconductor 293 hasbeen formed; the total thickness of this membrane structure is typicallyless than 4 μm. The dielectric layer 291 should closely match thecrystal lattice dimension and structure of the silicon 289; an exampleof such a dielectric with a similar crystal lattice is sapphire orundoped silicon under certain device design parameters. This MDIsemiconductor layer structure allows semiconductor devices to be formedon either side of the semiconductor layer 289 and remain electricallyisolated. An obvious application for this semiconductor or membranestructure is CMOS integrated circuits with p-type device formed on oneside and n-type devices formed on the opposite side. Low stressdielectric would be used to trench isolate individual semiconductordevices and complete the MDI process suggested by Method #2. Further,the second epitaxially grown semiconductor layer 293 can be a materialother than silicon such as GaAs or InP.

FIG. 12i shows a MDI circuit membrane composed of a silicon layer 295and a low stress dielectric layer 297. The dielectric layer 297 has beenpatterned to form open areas 299 into which semiconductor material wasdeposited by selective epitaxial growth. The selectively depositedsemiconductor 299 can be an semiconductor other than silicon, orsilicon. Semiconductor devices can be fabricated in the epitaxialislands 299 on the backside of the MDI semiconductor layer.

FIG. 12j shows a MDI circuit membrane composed of a silicon layer 295and a low stress dielectric layer 297. The dielectric layer 297 has beenpatterned and silicon islands 299 formed through selective epitaxialgrowth in the same manner as described for FIG. 12i. The silicon layer295 was subsequently patterned removing portions of the silicon layeropposite each island 299 that was epitaxially grown in the dielectriclayer, and then filling with low stress dielectric. Circuit devicesformed in the semiconductor islands 299 are dielectrically isolated.This membrane structure has similar applications as those suggested forFIG. 12h.

The structures FIGS. 12h, i & j show that a two level device structurecan be formed that permits more compact interconnection of the circuitdevices by the use of vertical interconnections. These structures alsosimplify the fabrication of mixed device technologies such as BiCMOS byallowing the Bipolar and MOSFET devices to be fabricated on either sideof the semiconductor membrane without an increase of interconnectcomplexity. The use of these MDI structures typically doubles the totalsurface area are available for interconnect metallization routing. Theobvious potential processing benefits can be a reduction in the numberinterconnect metallization layers per side and or relaxation of theinterconnect metallization pitch used, both of these benefits normallylead to an increase in circuit yield.

Dielectric Membranes as Sensor on Rigid Substrates

The mechanical (physical) and thermal properties of the nitride andoxide free standing membranes made with the Novellus equipment aresimilar to silicon membranes presently being used in the making of ICswith integrated sensors. Examples of such silicon membrane or diaphragmbased sensors are those made for sensing acceleration, pressure ortemperature, but not limited to these areas. The availability of suchoxide and nitride dielectrics for general application is novel with thisdisclosure.

The use of such free standing dielectrics membranes significantlyreduces the complexity of producing a membrane with non-conductingproperties in a silicon or semiconductor fabrication environment. Thelow stress dielectric membranes have selective etchants that allow thedielectrics to be processed independently in the presents of silicon.The known passivating capability and inert chemical nature of siliconnitride, and the relatively similar coefficient of thermal expansion ofsilicon, silicon dioxide and silicon nitride increases the operatingenvironmental range of sensors versus organic dielectrics.

Sensor diaphragms of low stress dielectric membranes produced on rigidsubstrates (typically semiconductor) can in most cases directly replacesilicon membranes or be used in conjunction with a silicon membraneformed on a rigid (conventional) semiconductor substrate. No newfabrication methods are required to integrate the low stress MDI processdielectrics into such sensor ICs.

APPLICATIONS OF MDI CIRCUIT MEMBRANES

The following discloses additional applications of the MDI process, andof forms of MDI circuit membranes and extensions of the MDI IC process.For example, a MDI circuit membrane with few or without active circuitdevices is called more specifically an interconnect circuit membrane.When the interconnect circuit membrane is used to interconnectconventional ICs in die form, the interconnect circuit membrane becomesa multiple chip module (MCM) interconnect circuit membrane as discussedimmediately below. This does not limit the application of a MDI circuitmembrane with active and passive circuit devices from being used in thefabrication of a MCM.

MDI Multi-Chip Module Interconnect Circuit Membrane

A MDI circuit membrane composed primarily of interconnect metallizationof one or several internal interconnect levels can be fabricated on asubstrate to provide electrical interconnections between the bond(signal) pads of various individual dice (ICs) as in the wellestablished packaging technique called multiple chip module (MCM). A MDIcircuit membrane made for this type of application is called multi-chipmodule Interconnect Circuit Membrane (ICM).

The multi-chip module interconnect circuit membrane is formed using theMDI process and conventional semiconductor processing techniques. Theinterconnect circuit membrane may have several thousands of die metalbond pad contacts or bonding points. The position of these metal contactor bonding points on the surface of the interconnect circuit membranecan be arbitrarily placed.

The multi-chip module interconnect circuit membrane can be formed on anysubstrate material that can meet the requirements for flatnessspecification, withstand the processing temperatures of the applied lowstress dielectrics), and can be selectively etched with respect to thelow stress dielectric(s) to form the circuit membrane. FIG. 13a shows anembodiment of an multi-chip module interconnect circuit membrane made asdescribed above by the MDI methods. FIG. 13a includes the interconnectcircuit membrane 320 with several internal metallization trace layers(not shown), ICs (dice) 322a, 322b, 322c, interconnect circuit membranesubstrate frame 324, and IC bonding contacts 326. FIG. 13b shows arelated structure which is a cross-section of a functional tester fortesting ICs with probe points 330 formed on attached ICs 336a, 336b.

The interconnect circuit membrane can also be formed (see FIG. 14) usingthe MDI method on a quartz substrate 340 that has been coated with arelease agent 342 such as KBr; this assumes that a single-crystal typemembrane or substrate 340 is not required for the fabrication of varioustypes of active circuit devices (SDs) in the interconnect circuitmembrane 320. Thus this structure is made as described above, butwithout monocrystalline SDs, as in a MCM that as a minimum requires onlyan interconnect circuit membrane or passive circuit devices andtransistors made from polycrystalline or amorphous semiconductor. FIG.15 shows the multiple chip module interconnect circuit membrane 320 ofFIG. 13a made with the release agent method of FIG. 14 and held by aseparately formed frame 350 that was bonded to the interconnect circuitmembrane 320 just prior to activating the release agent by scribing atthe shoulder of the quartz substrate, thereby cutting through theinterconnect circuit membrane 320 and applying the appropriate solventfor the release agent. The array of die (ICs) 322a, 322b, 322c can thenbe directly bonded (as described below) to the interconnect circuitmembrane 320, or the interconnect circuit membrane 320 can be alignedand mechanically held against the pads of the surfaces of the array ofdie (ICs) 322a, 322b, 322c to achieve electrical contact. Alternately,the circuit membrane made with the method utilizing polysilicon asdisclosed above and shown in FIG. 1j can also be used as an interconnectcircuit membrane to achieve a similar result as that shown in FIGS. 13a,13b and 15.

The interconnect metallization traces in the circuit membrane formelectrical connections from one or more of the contact pads of an IC toone or more contact pads of the other ICs. The diameter of these padscan vary from less than 0.5 mil (0.001" or 25 μm) to several mils. Thelimiting factors of the size of the IC contact pads is the lithographymethod used in fabrication and the assembly method; the cost of suchmethods increase as pad diameters less than 3 mils are employed.

A die can be bonded to a circuit membrane with any of severaltechniques. Techniques such as compressive metal to metal bonding withsuch metals as Indium or gold (or their respective alloys), infraredthermal bonding, laser bonding, and vertically conductive adhesive filmssuch as ZAF from 3M Corporation are examples. FIG. 16a shows thepackaging for a version of the structure shown in FIG. 13a, including ICcarrier lid 352, compressible material (such as silicone) 354 attachedthereto, carrier substrate 355 of the MCM package, contacts 356 to thecarrier signal pins 357. An alternative package is shown in FIG. 16b,with gastight gasket 358 enclosing pressurized volume 359.

If the interconnect circuit membrane 360 (see FIG. 17a) is made fromoptically transparent dielectrics such as low stress silicon dioxide andor silicon nitride, this permits the conventional die (IC) 362 to bevisually aligned on the interconnect circuit membrane 360 from thebackside or from the side of the interconnect circuit membrane 360opposite that of the die 362. A locally directed infrared or other heatsource (not shown) can then be applied from the backside of theinterconnect circuit membrane 360. On the opposite side (top side) isformed MDI pad 365 which is surrounded by solder wells 364, and solderweld bonds can be formed between the interconnect circuit membrane pads365 and the die bond pads 366 on which is formed solder bump 367 whichtypically is about 5 to 25 μm thick. The die 362 is attached face downagainst the interconnect circuit membrane 360.

Solder weld bonds are formed by using a heat source (not shown) such asa high intensity lamp, laser or metal instrument that can heat thesolder to its melting point. The heat source melts the solder 367 (seeFIG. 17b) directly under the die pad 360 by heating through theinterconnect circuit membrane 360 while the die is in contact with theinterconnect circuit membrane. The high temperature tolerance of thedielectric material, its innate elasticity and thin structure allow heatfrom the heating source to quickly reach and melt the solder 367 to forma bond of 3 to 10 μm thickness. The melted solder 367 wets both the diepad 366 and the interconnect circuit membrane pad 365, and when it coolsthe solder 367 contracts holding the die pad 366 firmly against theinterconnect circuit membrane pad 365. The solder bond is formed with nomore pressure than what would be exerted when the die 362 is brought incontact with the interconnect circuit membrane 360, pressing into theinterconnect circuit membrane and deflecting it by less than 25 μm. Whenthe solder 367 is melted, the interconnect circuit membrane 360 assumesa uniform planar contact against the surface of the die 362. This alsoremoves the requirement that the solder on each pad be formed to anexacting equivalent height.

The solder 367 (see FIG. 17c) alternatively is originally electroplatedonto the interconnect circuit membrane bond pad 365. The solder 367 iselectroplated to a height between 5 and 25 μm. The original height ofthe solder bump 367 is greater than the depth of the solder well 364 ofthe interconnect circuit membrane 360 in which the pad 365 of theinterconnect circuit membrane 360 is positioned. FIG. 17b shows thestructure of FIG. 17c after soldering or bond formation. If a die isremoved some excess solder 367 will remain on the pad 365 of theinterconnect circuit membrane 360. When a replacement die is bonded tothe same interconnect circuit membrane pad, any excess solder will flowor be pressed into the solder well 364 and not prevent the surface ofthe die 362 and interconnect circuit membrane 360 surface from achievinguniform firm contact with each other. A hermetic seal can be formedbetween the surface of the die 362 and the interconnect circuit membrane360. This is done by forming a metal bond along all edges of the die(not shown). This bond can be soldered, and is formed as describedbelow.

An advantage of the high temperature tolerance of the interconnectcircuit membrane which allows die solder bonding to be done without theapplication of any significant contact pressure between the interconnectcircuit membrane and die, is the ability (see FIG. 18 showing a top viewof bond pads on a die) to place die bond pads 370-1, 370-2, . . . ,370-k, . . . , 370-n directly over the circuit structures of the die(not shown). This allows the placement of semiconductor devices directlyunder a bond pad 370-k without concern that the semiconductor deviceswill be damaged, which is the case when mechanical wire bondingequipment is used to wire bond a die to its carrier. Damage-free bondingis not presently possible with current die wire bonding techniques,because such bonds are compressively formed with a mechanical arm whichpresses, rubbing through the die bond pad into the substrate of the dieto be placed anywhere on the surface of the die. The MDI method makesthe design of the IC easier, removes the need for reserved pad area onthe die (which saves 5 to 10% of the surface area of the die) does notrestrict the placement location of the pad, and allows the pads to belarger since area under the pads is no longer reserved exclusively forthem.

A hermetic seal bond pad 372 extends continuously along the perimeter ofthe surface of the die. A hermetic seal of the surface of the die withthe surface of the circuit membrane can be accomplished by the formationof an enclosing solder bond (comprising a solder well as described abovefor forming pad bonds) along the edge of the die. The hermetic seal istypically also a ground contact for the die.

The infrared solder bonding of die to the multiple chip moduleinterconnect circuit membrane is a simple process which also allows thedies to be easily removed and replaced without damage to theinterconnect circuit membrane. As shown in FIG. 19a, the IC 372 is heldby a vacuum tool 373 and aligned in close proximity to the MCMinterconnect circuit membrane 374. Once aligned, the interconnectcircuit membrane 374 is gently urged forward by a fluid pressure 376 adistance of several mils and the die 372 is urged toward theinterconnect circuit membrane 374 until contact is established. Theinfrared heat source 377 is applied to achieve localized heating of thesolder pads (not shown) on the IC die 372 until they melt and wet to theinterconnect circuit membrane 374.

The die 372 can be removed (see FIG. 19b) by using the same vacuum tool373 to gently pull on the IC 372 while localized heating from heatsource 377 of the IC is applied until the solder bonds melt and the IC372 can be pulled from the interconnect circuit membrane 374; no fluidpressure is applied to the interconnect circuit membrane 374.

The multi-chip module interconnect circuit membrane can be enhanced toinclude passive devices such as resistors, capacitors, and polysiliconor a-Si (amorphous-Silicon) TFTs (Thin Film Transistors). Such circuitelements can be fabricated into or onto the membranes due to the abilityof the dielectric membranes to tolerate processing temperatures inexcess of 400° C. These circuit elements can be fabricated internally tothe interconnect circuit membrane structure as part of various specificlayers or collectively on either external side of the interconnectcircuit membrane. After the interconnect circuit membrane is freed fromthe substrate on which it is initially formed, it is strong enough towithstand continued semiconductor processing steps.

The multi-chip module interconnect circuit membrane can further beenhanced to include any degree of active single crystal semiconductordevices. This is consistent with the earlier discussion of thecapability of bonding two or more MDI membranes to form a threedimensional IC, except in this case conventional dice (ICs) are bondedat the bond pads of the circuitry internal to a MDI circuit membranewhich is also serving the purpose of interconnect circuit membrane.

The multi-chip module interconnect circuit membrane provides novelcircuit cooling advantages due to its low mass structure. The thermalenergy generation of an IC is from the face side of the IC. ICs attachedface down against the circuit membrane allows the thermal energy of anIC to be radiated and conducted through the circuit membrane to a liquidor solid heat sink means on the opposite side of the circuit membrane.The circuit membrane provides a very short thermal path from the IC tothe heat sinking means. The ability directly to cool ICs in this manneris a unique feature of the MDI interconnect circuit membrane structure.The cooling efficiency that can be achieved by an IC that is fabricatedwith the MDI process as a circuit membrane is significantly improvedversus standard thickness substrates by the obvious reduction in thethermal mass resistance of the semiconductor substrate.

FIG. 13d shows in cross-section a portion of a MCM MDI interconnectcircuit membrane 329 including low stress dielectric and interconnectlayer 329a and semiconductor layer 329b where two die 331, 333 arebonded face down at their signal pads 335a, 335b, 335c, 335d. The MDIcircuit membrane 329 incorporates the pad drivers 337a, 337b, 337c, 337dthat normally would be incorporated onto the die (IC). The primaryadvantages of integrating the IC pad drivers into the MCM are circuitperformance, simplified use of mixed device technologies (Bipolar &CMOS), cooling and die size (IC real estate).

IC pad drivers are typically designed to meet a broad range of circuitdesign conditions which often results in a trade-off of performance. Thedesigner of the MCM pad driver has a better understanding of theoperational requirements of the pad driver, and therefore, can typicallyoptimize the pad driver design for higher performance. Pad drivers inthe MCM can be formed from Bipolar transistors while the IC can be CMOS.This achieves many of the benefits of BiCMOS without the fabricationcomplexity. The pad drivers of an IC typically generate more than halfof the thermal energy of the IC especially at high operating speeds.Placing the pad drivers in the MCM provides a means to bring the primarythermal components of an IC into direct contact with heat sinking means.IC pad drivers are typically the largest transistor structures making upthe IC. Moving the pad drivers of the IC into the MCM can result in apotential 5-10% reduction in the size of the overall IC.

The incorporation of low complexity ICs into a MCM MDI circuit membranecan reduce overall MCM assembly and parts costs. Such often usedcircuits as bus drivers or combinatorial logic can be incorporated intoa MCM reducing parts and assembly costs without a corresponding increasein the cost to fabricate the MCM. (Once the decision has been made toinclude such circuits in a MCM MDI circuit membrane, there is nosignificant cost difference between the inclusion of 1,000 transistorsor 10,000.) Yield of circuits in a MCM circuit membrane can be addressedthrough redundant fabrication of the desired circuit devices; thecircuit devices of the MCM circuit membrane comprise only a small amountof the total surface area, typically less than 10%. The circuit devicesof the MCM circuit membrane can be tested for defects. A MDI functionaltester membrane disclosed herein, can be used to perform this testingand, in the same step if so desired, to blow anti-fuses to enable thedefect free circuits or fuses to disable the defective circuits.

The multi-chip module interconnect circuit membrane, like prior art PCBs(printed circuit boards), can provide access to the contact pads of theIC from the opposite side on which the IC is attached. This accesspermits in circuit testing of the bonded IC to be performed by aseparate interconnect circuit membrane fabricated to do such afunctional test, and also permits electrical continuity testing of thetraces fabricated in the multi-chip module interconnect circuitmembrane, again performed by an interconnect circuit membrane testsurface fabricated for that purpose. FIG. 20 is a top side view of aninterconnect module showing an interconnect circuit membrane surfacebond pad 380 and interconnect circuit membrane feed-through contacts 382from the bond pads of the dice 386-1, 386-2, 386-3 mounted bycompression or solder bonding (see FIG. 21) on the backside of theinterconnect module of FIG. 20. Also shown in FIG. 21 are the backside388 of the etched silicon substrate and the interconnect circuitmembrane surface 190 with the traces 392-1, 392-2, 392-3 interconnectingthe dice 386-1, 386-2, 386-3.

The multi-chip module interconnect circuit membrane method could also beused to construct a functional IC tester surface with a high number ofprobe points (in excess of several thousand) for testing ICs while inwafer or die form (i.e., wafer or die sort testing), and also to wafersort subsections of ICs such as an ALU (Arithmetic Logic Unit), FPU(Floating Point Unit), cache segment, etc.

A functional circuit membrane tester surface made with the MDI processoffers advantages not presently available from present on-wafer ICmembranes testers constructed from polymers such as continuous hightemperature operation (greater than 100° C.) without deleterious effectsdue excessive material expansion or at-speed IC testing without EMinterconnect trace coupling.

High numbers (in excess of 1000) of pads or traces of an IC or ICsubsection can be contacted. The contact sites of the IC (die) can bearbitrary in position and all sites can be contacted at one time. Thecontact sites (pads or traces) can be less than 50 μm (2 mils) or evenless than 2 μm in diameter. The probe points of the functional testersurface can be less than 50 μm or even less than 1 μm in diameter, andcan be placed on center to center distances of less than twice thediameter of the probe points. (Functional probe points can be formed tobe 12 μm in diameter and spaced on center to center distances of lessthan 24 μm.) The ability to form and contact high numbers of pads ortraces with dimensions smaller than presently available in the industryis due to the ability of the MDI IC process to form multiple layers ofinterconnect with established semiconductor fabrication means resultingin a flexible and elastic membrane structure. The multiple layers ofinterconnect provide for denser contact test sites, the thin membranestructure provides for contact conformation with little required force(typically less than 10 psi), and the compatibility of the dielectricmaterial with standard IC fabrication techniques allows contact ofcontact site dimensions less than 2 μm.

A MDI circuit membrane 332 used as an IC functional tester surfaceextends under fluid pressure "P" to make contact with the surface of awafer or single die 339 to be tested as shown in FIG. 13c. The MDIcircuit membrane 332 can be formed with diameters of approximately thesize of the substrate on which it is formed and to be extended more than40 mils (0.1 cm) depending on the diameter of the membrane. Probe points330 on the tester surface are not restricted in size, and can range insize from greater than 4 mils (0.01 cm) to less than 2 μm in diameter.The number of probe points 330 can vary from less than 100 to severalthousand without restriction and the placement of the probe points canbe arbitrary. These capabilities of the functional tester surface areprimarily due to the structural formulation of the materials used andthe fabrication methods of the MDI process. The MDI functional testersurface can rub through the native aluminum oxide layer of the bondingpads of the die 339 being tested by laterally moving the wafer or die339 holding mechanism several microns (typically less than 10 μm) in aback and forth motion. This is uniquely facilitated by the durability ofthe circuit membrane, the surface flatness of the membrane, theuniformity of probe point height and the low pressure required to extendthe membrane surface so that all probe points are in contact with thesubstrate.

In the case of sub-section testing of a die, the detection of adefective sub-section before packaging permits the option of pinning-outthe IC as one of several alternative product configurations, andthereby, making useable an otherwise scrapped IC.

In FIG. 13b the multi-chip module interconnect circuit membrane hassignal pad probe points 330 formed on the side opposite of theattachment of dice 336a, 336b. The dice 336a, 336b of the multiple chipmodule provide functional test signals to the probe points 330. Thetesting performed by the multiple chip module can be functional testingof a whole die or a subsection of the IC. The probe points 330 arealigned over a die (not shown) to be tested on a wafer, and theinterconnect circuit membrane 332 is extended several mils (nominally15-30 mils) by a fluid pressure as shown in FIG. 13c. The wafer israised to make contact with the probe points 330, the functional testperformed, the wafer is lowered, positioned to the next IC (die locationon the wafer) and the functional test repeated until all the ICs on thewafer have been tested. Testing die in this manner can also be done ondie that has already been cut from a wafer. This is done by aligning andholding the individual die in contact with the probe points and thefunctional test performed. Piezoelectric horizontal vibration applied tothe interconnect circuit membrane 332 or the wafer or individual diewhile the functional testing is being performed can be used to rubthrough native metal oxide on the die pads.

Alternately the native metal oxide (approximately 30 Å thick) on thepads of a die can be rubbed through by the probe points of thefunctional tester surface by repeatedly moving the wafer or die severalmicrons in a lateral manner when the tester surface is first broughtinto contact with the IC to be tested. The mechanical control presentlyavailable with most wafer or die handling equipment is sufficient toeffect this rubbing action.

The interconnect circuit membrane can range in thickness from less than2 μm to over 25 μm. A typical thickness per layer of interconnect(dielectric and metal traces) is 1 to 4 μm. Dielectric layer of severalmicrons thickness may be used as a planarizing technique to lessen metaltrace step height differentials that can develop when more than twointerconnect layers are used. Planarization of metal traces can also beachieved by depositing the dielectric to a thickness inclusive of themetal trace. A channel that is at least as deep as the desired thicknessof the metal trace is patterned in the dielectric. Then establishedlift-off techniques are used to fill the channel with metal.

FIGS. 22a to 22c show this process. In FIG. 22a, a portion of theinterconnect circuit membrane dielectric 400 is shown with patternedresist layer 402 overlying. Recess 406 is then formed in dielectric 400by isotropic etching to form undercut portions 404 equal nearly to thedepth of etched recess 406. In FIG. 22b metal layer 408 is deposited. InFIG. 22c lift off stripping of resist 202 is performed leaving onlymetal trace 408, and the next dielectric layer 410 is deposited.

Planarization of the interconnect structure can be achieved by applyinga thick layer of a planarizing polymer over the MDI dielectric film. Thepolymer must have an etch rate similar to that of the MDI dielectricfilm. The polymer is then removed completely by dry etch means and inthe same process removing any dielectric surface features that extendedinto the polymer layer. This planarization process is identical to theoptional planarization process disclosed in the process steps for theAir Tunnel. The only differences being the material being planarized isa MDI dielectric and not a-Si.

The ability of the interconnect circuit membrane to achieve reliablecontact on the bond pads of a die (see FIG. 13b) is strongly influencedby the thickness of the interconnect circuit membrane 332 which containsthe probe points 330 for contacting the die to be tested. Aninterconnect circuit membrane 332 may be 25 μm thick or more as requiredfor example by transmission line design considerations of interconnecttraces. Interconnect circuit membrane thickness at the probe points ispreferably less than 8 μm. The structure of FIG. 13b can be fabricatedas shown in FIGS. 23a and 23b.

In FIG. 23a an etch stop layer 412 (typically metal) is deposited at acertain thickness of the interconnect circuit membrane 332 as theinterconnect circuit membrane 332 is fabricated from repeatedapplications of dielectric and metallization layers to a thickness of 8to over 25 μm. After the fabrications of the interconnect layers of theinterconnect circuit membrane 332 is completed, an area over the probepoints 330 is masked and etched to the etch stop layer 412 (see FIG.23b). The etch stop layer 412 is then etch removed. Alternatively, thearea 414 over the probe points 330 can be etched free of overlyinginterconnect layers as each layer is itself patterned as part of thefabrication of the interconnect metallization. Then dice 336a, 336b areattached to the interconnect circuit membrane 332.

MDI Source-Integrated Light Valve (SLV) Direct Write Lithography Tool

The MDI process technology can be used to fabricate a circuit membranewith an nxm cell array of active radiation sources of X-ray, DUV (DeepUltra Violet) or E-Beam sources that can be used to form lithographicpatterns in a film of appropriately sensitive resist. The preferredembodiment in this disclosure uses a X-ray source. Variations of theelectrodes, gas contents and structure of the radiation source cell canbe made within the overall structure of this source integrated lightvalve embodiment. The preference for the X-ray source embodiment is itscapability to pattern a smaller feature size.

The radiation source cells in the membrane are under computer control,so that this type of MDI membrane combines the functions of reticle ormask and a light or radiation source for fabricating ICs. The radiationsource cells of this MDI circuit membrane are positioned (aligned) overan area of the substrate to be patterned and at a uniform distance fromor in contact with the substrate. Each cell then illuminates (exposes)various portions of the substrate directly beneath it as required. TheMDI circuit membrane functioning in this manner is called herein aSource-Integrated Light Valve (SLV) and can generally be categorized asa Direct Write (maskless) Lithography Tool.

The array of Radiation Source Cells (RSC) that make up the patterningstructure of the SLV, emit a collimated source of radiation. Theexposure feature size of this collimated radiation source isdimensionally much smaller than the area the RSC occupies on the surfaceof the SLV. The emitted collimated radiation source is referred to asthe Radiation Exposure Aperture (REA). The SLV is moved in a scanningX-Y manner over the substrate to be patterned. The dimensions of thescanning motions cause each REA of an RSC to pass over a portion of thesubstrate that typically is equal in area to the size of the RSC. TheRSCs are operated in parallel, and the patterned areas on the substratebeneath each RSC adjoin each other and collectively form a much largeroverall pattern on the substrate. This pattern can be as large as theunderlying substrate. The scanning motion of the RSCs is generated bywell known computer controlled mechanical motor driven stages orpiezoelectric driven stages. The SLV or the substrate can be attached tothe motion stage. Due to the short X-Y motions in the use of the SLV,and the desire to scan the RSCs with REAs of dimensions much less than 1μm and in some cases with dimensions of less than 50 nm, piezoelectricmotion stage drive is the preferred embodiment. As the REAs of the SLVcircuit membrane are scanned over the substrate, they are turned on oroff by computer controlled logic associated with each RSC to effect thedesired exposure pattern to be made on the surface of the substrate. Anexample would be a RSC with an area of 25 μm by 25 μm and a REA of 0.1μm in diameter. In order for the REA to expose an area the size of theRSC, a scanning motion of 25 μm by 0.1 μm would be made 250 times in theX-axis direction while translating 0.1 μm in the Y-axis direction aftereach scan.

FIG. 24 shows a overhead (plan) view of a prototypical SLV 420 with anarray size of 4,096 by 4,096 RSCs, and an overall size of approximately5 inches by 5 inches. The SLV array 420 of RSCs has associated controllogic 424 for loading the pattern data to each RSC. The SLV 420(including the control logic 424) is a MDI circuit membrane of nominally8 μm to 50 μm in thickness made by established semiconductor processesand held in a rigid frame 426 made from the original substrate on whichthe SLV 420 was fabricated or a frame bonded to the SLV prior to theselective etch removal of the substrate on which the SLV (MDI circuitmembrane) was fabricated.

Electromagnetic coupling alignment structures 430 (described below) areshown also.

FIG. 25 shows a cross-section of two RSCs 434-1, 434-2 in a SLV 440. ASLV 440 in the preferred embodiment has rows and columns of RSCs 434-1,434-2 where each row and each column can include more than 1,000 RSCsresulting in a total count of RSCs in a SLV in excess of severalmillion. FIG. 25 shows a MDI circuit membrane 440 which is 8 to 50 μmthick, data bus interconnect metallization 442-1, 442-2, control logic424-1, 424-2, X-ray sources 446-1, 446-2, and REAs 434-1a, 434-2a.

FIGS. 26 and 27 are cross-sections of two different potentialimplementations of X-ray RSCs 434. FIG. 26 uses a metal cathode 450 witha highly textured surface 452 in the shape of adjacent cubes; eachcorner of the extended portion of each cube is a cold electron pointemitter which emits electrons into the electric field 456 created by thehigh voltage x-ray emitting target 458 beneath it. The size of the RSC434 is approximately 25 μm ×25 μm. The thickness (height) of the RSC 434is approximately 8 to 50 μm. The materials used to fabricate the SLV(and RSCS) are those employed to fabricate a MDI circuit membrane asdescribed above--metal 459, low stress dielectric 460 and a singlecrystalline semiconductor membrane substrate 472. The processing stepscan vary widely, but are those used in conventional semiconductor andmicro-machinery fabrication.

The textured cathode 450 will typically be in the shape of a squarearray of rows and columns of metal cubes, with each cube havingdimensions much smaller than the overall dimensions of the squarecathode layer. As an example, if the cathode 450 is 4 μm² in area, thedimension of each cube would be 0.1 to 0.5 μm on a side. The texturedcathode cubes are made by established lithography techniques (E-Beam orDUV optical lithography) and anisotropic RIE processing. This cathode450 configuration is a novel part of the SLV because of high density ofcold electron emitters, and therefore, high electron fluence onto theanode 458. The cathode 450 emits electrons which are accelerated by theelectric field 456 created by the voltage potential of the target oranode 458. When electrons reach the target 458, X-rays are emitted fromthe target 458. X-ray absorber materials 466 such as tungsten or goldare deposited as layers as part of the fabricated structure of the SLVmembrane to provide a means to form the REA 470 and in back scatterlayer 461 to prevent back scattered X-rays from reaching control logicdevices 472 with interconnect 473 to other RSCs in the SLV. This RSC isrelated to the well known X-ray vacuum tube design. The space 474separating the cathode and target is in partial vacuum. This separationdistance can vary from less than 1 μm to 40 μm.

FIG. 27 shows an alternative RSC 434 that uses a laser diode 480 toirradiate onto a target 458 which in turn emits X-rays. The laser diode480 and the target 458 are separated by a distance "d" of 1 or moremicrons in a cavity 482 that is optionally a partial vacuum. Except forthe technique of inducing the emission of X-rays from the target 458material, the RSCs 434 shown in FIGS. 26 and 27 are functionally thesame. RSCs that employ gases such as Cd-Hg or Xe-Hg as in the case ofDUV radiation sources, the structure of the electrodes in the cavity ofthe RSC would change and X-ray absorber layers would not be necessary.RSCs that emit free electrons for resist patterning would require thatthe REA 470 be an opening or that it be closed by a material thin enoughto allow the accelerated beam of electrons to pass through it. If theREA 470 is an opening from the electrode cavity 482 of the RSC, then theSLV would have to be operated in vacuum, just as is the case with E-Beamlithography tools. The Hampshire Corporation (Marlborough, Mass.)presently manufactures an X-ray lithography tool which uses a laserdiode stimulated X-ray radiation source.

Although laser diodes that emit radiation with a wavelength less thanapproximately 6,000 Å do not presently exist, development of shorterwavelength laser diodes is on-going and contemplated as a radiationgenerator for the SLV. There is significant demand for diodes withshorter wavelengths for use in consumer and computer peripheral productslike compact discs. Such devices in the not too distant future may comeinto existence and such laser diodes could possibly be integrated intothe SLV as a radiation source generator of a RSC. The use of laserdiodes is therefore contemplated. An additional application of the SLV(with a RSC of appropriate radiation capability) is radiation-inducedCVD (chemical vapor deposition). The use of an SLV in this mannereliminates the need for mask and etch processing steps. The material ofinterest to be deposited, be it metal or dielectric, is deposited in apatterned form. This provides for a maskless, resistless, etchlessprocess and it has significant cost and particulate contaminationadvantages. The SLV operating in this manner supplies radiation at thesurface of a substrate where a mixture of gaseous compounds are present.This process step is mechanically the same as exposing a resist layer onthe substrate as discussed above. The radiation from a RSC causes thecompounds to react at the surface of the substrate leaving a depositionof material selectively on the irradiated portions of the surface of thesubstrate. This method of radiation-induced selective CVD has beendemonstrated with the use of excimer lasers. Laser diodes that can emitradiation in the excimer frequency band of 75 to 250 nm do not presentlyexist.

MDI Source-External Particle Valve (SPV) Direct Write Lithography Tool

The MDI process technology can be used to fabricate a circuit membranewith an nxm cell array of individually controlled electro-static orelectromagnetic valves or shutters. These valves can be used to controlthe passage of charged particles through the circuit membrane to createan exposure pattern in a film of appropriately sensitive resist. Thearray of valves on the MDI circuit membrane is for practical purposesthe same structure as the SLV structure shown in FIG. 24. The primarydifference is the use of particle valve cells for patterning an externalparticle source which illuminates one side of the SPV circuit membrane,instead of a cell with an integrated patterning source as in the case ofthe SLV lithography mask.

The array of valve cells of this MDI circuit membrane is placed betweena collimated ion particle source and at a uniform distance from or incontact with a substrate coated with a thin film of resist. The array ispositioned (aligned) over an area of the substrate to be patterned andthe array of valves, under individual computer control, allow ion(charged) particles to expose or not expose an area of resist coatedsubstrate directly under beneath it. The MDI circuit membranefunctioning in this manner is called herein a Source-External ParticleValve (SPV).

The collimated particle beam passed by a particle valve has a fixedexposure feature size called the Particle Exposure Aperture (PEA). TheSPV is moved in a scanning X-Y manner over the substrate to bepatterned. The dimensions of the scanning motions cause each PEA of aparticle valve to pass over a portion of the substrate that typically isequal in area to the size of the valve. The valves are operated inparallel (simultaneously), the patterned areas on the substrate beneatheach valve adjoin each other and collectively form a much larger overallpattern on the substrate. The scanning motion of the valves is generatedby well known computer controlled mechanical motor driven stages orpiezoelectric driven stages. The SPV or the substrate can be attached tothe motion stage. Due to the short X-Y motions in the use of the SPV,and the desire to scan the valves with PEAs of dimensions much less than1 μm in diameter and in some cases less than 50 nm, piezoelectric motionstage drive is the preferred embodiment. As the valves of the circuitmembrane are scanned over the substrate, they are turned on or off bycomputer controlled logic associate with each valve to effect thedesired exposure pattern to be made on the surface of the substrate. Anexample would be a valve with an area of 25 μm by 25 μm and a PEA of 0.1μm in diameter. In order for the valve to expose an area the size of thevalve, a scanning motion of 25 μm by 0.1 μm would be made 250 times inthe X-axis while translating 0.1 μm in the Y-axis after each scan.

FIG. 24 (also a plan view for the SLV) shows an overhead (plan) view ofa prototypical SPV 420 with an array of 4,096 by 4,096 ion or chargedparticle valves, and an overall size of approximately 5 inches by 5inches. The SPV array 420 of valves has associated control logic 424 forloading the pattern data to each valve. The SPV 420 (including controllogic 424) is a MDI circuit membrane of nominally 4 μm to 8 μm inthickness made by established semiconductor processes and held in arigid frame 426 made of the original substrate on which the SPV 420 wasfabricated. Electromagnetic coupling alignment structures 430 (describedbelow) are shown also.

FIG. 29a shows a cross-section of two valves 502, 504 in a SPV 506. ASPV in the preferred embodiment has rows and columns of valves 502, 504where each row and each column can include more than 1,000 valvesresulting in a total count of valves in a SPV in excess of severalmillion. FIG. 29a shows a MDI circuit membrane 508 which is 4 μm to 8 μmthick, data bus 510a, 510b, control logic 512a, 512b and PEA (aperture)514a, 514b for passage of ion particles. FIG. 29b shows a plan view ofthe valves 502, 504 of FIG. 29a.

The valves of the SPV prevent the passage of ions or charged particlesby using an electric or magnetic field of the same polarity to deflectapproaching particles from the PEA. The PEA 514a, 514b is typically asquare opening in the SPV circuit membrane 508. The PEA 514a, 514b iscircumscribed by a metal film 520a, 520b which creates a localelectro-static field over the PEA by applying a voltage potential, or bya metal wire 540 (coil) (shown in FIG. 29e) which creates a localelectromagnetic donut shaped field. When the SPV is flooded with acollimated source of charged particles, the particles that fall upon thePEA 514a, 514b will pass through the SPV 506 and continue onto theresist layer (not shown) to be patterned directly beneath the SPV 506.If a electro-static field or electro-magnetic field of sufficientstrength and of the same polarity as the ionic particle is present, theionic particle will deflect away from the PEA and strike some otherportion of the SPV surface. The PEA is small in area relative to thetotal generated electric field volume, therefore, only a smalldeflection of the approaching charged particle is required for it tomiss the PEA and strike the surface of the SPV.

The SPV 506 is typically operated in an enclosure (not shown) undervacuum. The charged ions or particles can be of any material that can beappropriately accelerated or caused to uniformly flood the surface ofthe SPV. Some examples of charged particles that have been used assources for patterning resist films are electrons (negative), protons(positive) and gallium+ (positive). The SPV can be in close proximity orcontact with the substrate to be patterned, or it can be positionedbefore imaging lens elements of the lithography equipment that could beused to enhance the focus of the particles beams formed by passingthrough the SPV circuit membrane or demagnify (reduce) the imagegenerated by the SPV.

FIGS. 29c and 29d are cross-sections that show different methods forimplementing an electro-static shutter or valve of the SPV. FIG. 29cshows a PEA 514a formed by an anisotropic etch of the monocrystallinesemiconductor membrane 524; <100> monocrystalline semiconductor wetetches with a wall angle of approximately 54° or dry etching (RIE)methods can also be used to etch a round or square hole to form the PEA514a in either the low stress dielectric 526a, 526b or semiconductorlayer 524 of the SPV circuit membrane. The via in the low stressdielectric membrane 526b on the backside of the semiconductor membranewas formed after the wet etch of the PEA 514a. A thin metal film 528 oftypically less than 2,000 Å is deposited by sputtering or CVD means. Themetal film 528 circumscribes the PEA 514a and has a diameter typicallyof less than 4 μm, and the PEA typically has a diameter d of less than500 nm. FIG. 29d shows a PEA 530 formed by etching a via in a flat metalfilm 532 that was deposited on the semiconductor membrane 534. The metalfilm 532 forming the PEA in this example is typically less than 2,000 Åthick and has a diameter of approximately 4 μm. Also shown are lowstress dielectric layers 536a, 536b.

FIGS. 29e and 29f show a method for implementing an electromagneticshutter or valve of the SPV. FIG. 29e is a plan view of the structure ofFIG. 29f. Shown are semiconductor membrane layer 542 and layer 544 lowstress dielectric. A metal coil 540 is formed on MDI circuit membrane508 so a current loop can be created around the PEA 514a. The PEA 514ais formed by wet or dry anisotropic etching in either the dielectric orsemiconductor portions of the MDI circuit membrane 508. The metal line540 is typically less than 2 μm wide, less than 1 μm thick andcircumscribes a area "a" typically less than 4 μm in diameter. The PEA514a is smaller than the inner diameter "a" of the loop of the metalline 540 and typically less than 500 nm.

Control logic is associated with each valve of a SPV, supplying voltageor current as necessary to the metal structure circumscribing the PEAs.In FIGS. 29c and 29d the valve control logic applies a voltage potentialto the metal film 528, 532 respectively circumscribing the PEA. If theapplied voltage potential is of the same polarity as the chargeparticles striking the surface of the SPV, the particles will bedeflected from passing through the PEA. This is the valve closedcondition. If the applied voltage potential is of the opposite polarityor zero, particles will pass through the PEA. In FIG. 29e and 29f thevalve control logic applies a current through the metal wire 540circumscribing the PEA 514a. Depending on the direction of the currentin the wire 540 (clockwise or counter-clockwise about the PEA) amagnetic field with a polarity is generated. If the field polarity isthe same as the approaching charged particle it will be deflected, andif the field polarity is the opposite or zero particles will passthrough the PEA.

The design of electro-static or electromagnetic valves for achieving thedesired functions of the SPV can vary widely. The examples presentedhere are not intended to limit the scope of the design structure of suchvalves for use in a SPV.

MDI Mechanical Light Valve (MLV) Direct Write Lithography Tool

FIGS. 29g through 29k show portions of a direct write lithography toolwhich uses repeated micro-machined mechanical electro-static shuttercell 550-k for the patterning of photonic or particle exposure source.The overall structure and function of this lithography tool is similarto the above described SLV and SPV tools. It is composed of rows andcolumns of pattern forming elements called shutter cells 550-k that areeach approximately 25 μm by 25 μm. FIG. 29g is a plan view of severalsuch cells 550-1, 550-2, . . . , 550-k, . . . , 550-n. FIG. 24represents a general plan view of the whole lithographic tool, whichlooks the same in plan as the SLV and SPV tools.

An array of shutter cells 550-k made on a MDI circuit membrane is placedbetween a collimated photonic or ion particle source and a substrate tobe patterned, and can be used in projection or contact lithographymethods to pattern such a substrate coated with a thin film of resist.The array is positioned (aligned) over an area of the substrate to bepatterned and the array of shutter cells, each under individual computercontrol regulating the shutter opening or REA (Radiation ExposureAperture) through the circuit membrane, exposes an area of the substratedirectly beneath or corresponding to it. The MDI circuit membranefunctioning in this manner is called herein a Mechanical Light Valve(MLV).

The size of the REA collimated exposure image passed by a shutter cell550-k is variable. Further, the REA does not need to be a physicalopening through the MDI circuit membrane for photonic sources, but onlytransparent to the incident photonic radiation. The MLV is moved in ascanning X-Y manner (shown by X-Y axis 554) over the substrate to bepatterned. The dimensions of the scanning motions cause each REA of ashutter cell to pass over a portion of the substrate that typically isequal in area to the size of the shutter cell 550-k. The shutter cells550-k are operated in parallel (simultaneous) manner; the patternedareas on the substrate beneath each shutter cell adjoin each other andcollectively form a much larger overall pattern on the substrate. Thescanning motion of the shutter cell is generated by well known computercontrolled mechanical motor driven stages or piezoelectric drivenstages. The MLV or the substrate can be attached to the motion stage.Due to the short X-Y motions in the use of the MLV, and the desire toscan the shutter cells with REAs of dimensions much less than 1 μm indiameter and in some cases less than 50 nm, piezoelectric motion stagedrive is the preferred embodiment.

As the shutter cells of the circuit membrane are scanned over thesubstrate, they are opened or closed by computer controlled logic 556-kassociated with each shutter cell 550-h to effect the desired exposurepattern to be made on the surface of the substrate. An example would bea shutter cell with an area of 25 μm by 25 μm and a REA setting of 0.1μm in diameter. In order for the REA to expose an area the size of theshutter cell, a scanning motion of 25 μm by 0.1 μm would be made 250times in the X-axis while translating 0.1 μm in the Y-axis after eachscan. The size setting of the REA is typically fixed for a given scan ofthe substrate, and vector motion of the REA can be used to improveexposure performance when there are areas of the substrate common to allshutter cells that do not require exposure such as is the case with viapattern layers.

The operation of the shutter cells regulates the opened/closed status ofan opening that allows a fluence of the exposure source (photons orparticles) to pass, and in the same operation dynamically determines theexposure CD for the lithography tool. The ability to change the minimumexposure CD or REA of the MLV is unique to this tool, versus the SLV andthe SPV which have fixed exposure apertures. The ability to dynamicallychange the size of the REA allows the pattern or geometry fracturedatabase of the IC to be partitioned into CD specific geometry sets perpattern layer of an IC. This provides the capability to optimize theperformance of the lithography tool by patterning those portions of anexposure image with the smallest CD (processing rate is due to the sizeof the REA, therefore, the smaller the REA the greater the number of REAexposures), separately from other portions of the exposure imagecomposed of larger CDs.

FIGS. 29h and 29i show plan views of two different shutter armstructures of a shutter cell 550-k. FIG. 29i shows a spring suspensionstructure of the shutter arm 560, 566. The shutter arm 560, 566 isformed of metal or a combination of metal and low stress dielectric andis suspended over the surface of the substrate from two electricalcontacts 562, 564 on either side of the shutter 566. The shutter arm560, 566 can move (in the direction 570-Y indicated) by the switchedapplication of electrostatic potentials. The REA 572 of the shutter cellis formed by the intersection of a square opening 574 through thenon-transmissive material of the shutter arm and a similar openingthrough a metal electrode film directly below the central area of theshutter 566 and fixed to the surface of the substrate. This opening 574is typically less than 2 μm in diameter, and is positioned with a 90°rotation relative to the long axis of the shutter arm 560. FIG. 29hshows in dotted outline 578 as an alternate position of the shutter arm560, 566 produced by the application of electro-static potentials. Thesize of the REA is determined by the projected intersection of theopening in the central area 566 of the shutter and the opening in thefixed metal film beneath it. The REA of the shutter 572 is closed bymoving the central area of the shutter arm 566 a distance greater thanthe diagonal length of the square opening 574.

The shutter arm 560, 566 is positioned or moved by electro-staticpotentials. The shutter arm 560, 566 is provided with a voltagepotential. Electrodes 580a, 580b positioned at either end of the maximumtravel displacement of the central area of the shutter arm 566 areselectively set at opposite potentials sufficient to attract or repelthe shutter arm a desired displacement. The shutter arm is held inphysical contact with the electrode film beneath it by applying anopposite potential to the lower substrate electrode (not shown).

FIG. 29j is a cross section of part of the shutter cell through line j-jof FIG. 29h. FIG. 29k is a cross section along line k-k of FIG. 29j.

As shown in FIG. 29j, the shutter cell is fabricated with establishedsemiconductor processing methods. The array and shutter cell controllogic are fabricated on a MDI circuit membrane 590. A trench 592 isformed down to a lower dielectric layer 590 of the circuit membranesufficient in size to accommodate the motion of the central area 566 ofthe shutter arm. The electrode 594 under the central area 566 of theshutter arm is formed, and a conformal sacrificial layer (not shown) ofa-Si (amorphous Silicon) is deposited over the area of the shutterarray. The thickness of the a-Si layer determines the separationdistance that the shutter arm is suspended above the circuit membrane.Into the a-Si layer via openings 562, 564 (FIGS. h and i) are patternedto the electrode contacts of the shutter arm 560 and to the electrodecontacts of the shutter arm positioning electrodes 580a, 580b. A metallayer 600 optionally combined with a dielectric layer 602 is depositedand patterned to form the shutter arm 560, 566 and the positioningelectrodes 580a, 580b. The opening 574 that is used to form the REA isalso patterned and etched via RIE processes. This opening 574 as aminimum passes through the shutter arm and the electrode 594 below thecentral area of the shutter arm that is used in forming thecross-sectioned area that is the REA 572. The opening 574 may becompletely through the MDI circuit membrane 590, or stop on thedielectric layer depending on the transmission requirements of theexposure source. The a-Si is then selectively removed which leaves theshutter arm 560 free standing and connected to the circuit membrane atonly two electrode contact sites 562, 564 (FIGS. h and i). The shutterarm 560, 566 is free to move in two directions 570-Y, 570-Z based on thevoltage potentials applied to the shutter arm 560, 566 positioningelectrodes 580a, 580b, and the substrate electrode 594 beneath thecentral portion of the shutter arm.

Fixed Free Standing Membrane Lithography Masks

The MDI process can be used to form free standing membranes used aslithographic masks with fixed patterns. The MDI process can be used toform optical, X-ray and stencil masks (ion or charged particle masks).The mask substrate is made from a low stress dielectric and optionallysemiconductor material. In the preferred embodiment masks are made fromoxide and nitride low stress dielectric made on the Novellus equipmentand consistent with the formulation presented above or as variations ofsuch low stress membranes formed on other oxide and nitride fabricationequipment. The patterning material is a semiconductor such as silicon,another dielectric material or a metal which is non-transmissive to aphotonic exposure source. If the exposure source is an ion or chargedparticle beam, then the patterning is accomplished with voids (openingsthrough the membrane) or a lack of material, or stencil.

FIGS. 29l and 29m show in cross-section the formation of a fixedphotonic mask. A membrane substrate 620 initially formed of dielectriclayers 622, 624 and silicon 626 (patterning material shown with pattern)is formed as described above. The thickness of the dielectric layers622, 624 of the membrane is typically 2 to 3 μm thick, and the silicon626 may vary in thickness depending upon the desired sidewall angle ofthe silicon or the CD aspect ratio (ratio of layer thickness to the CDopening of the pattern). The silicon layer 626 can be patterned directlywith established resist spin-on films, pattern generation lithographytools such as E-beam and selective silicon etch processing.

FIG. 29l shows in cross-section a patterned silicon layer 626 formed byfirst patterning an overlay of metal 630 such as tungsten, and thenselectively etching the silicon layer 626 done to the dielectric layer622. A metal pattern could also be formed by removing the silicon layeraltogether and depositing a metal film, then subsequently patterning it,or by selectively undercutting the patterned silicon layer of FIG. 29lto employ a lift-off process with a noble metal such as gold.

FIG. 29m shows in cross-section the structure of FIG. 29l after thedeposition of a low stress dielectric membrane layer 632 over thepattern and the removal of the dielectric layers 622, 624 below thepattern. The deposition of low stress dielectric membrane layer 632 overthe pattern forms a structural membrane layer so that the original lowerdielectric structural layers 622, 624 could be removed.

In the case of X-ray masks gold is often used as the patterning filmbecause it is a good X-ray absorber. Gold and the noble metals ingeneral can be patterned only with wet processing techniques and or incombination with electro-plating techniques. The silicon pattern 626 ofFIG. 29l can be dry etched (RIE) to achieve significant aspect ratios.If the silicon layer 626 is heavily gold doped prior to RIE processing,this method of mask formation can be used as an X-ray mask without CDfeature size or aspect ratio limitations of wet etch processing.

FIG. 29n and 29p show a stencil mask formed by patterning the dielectriclayer 640 of a MDI silicon 642 and dielectric 640 membrane with a RIEprocess. The silicon layer 640 is then selectively wet etch removed toleave a stencil pattern as shown by the openings in the dielectricmembrane 644l, 644b, 644c in FIG. 29p. This sequence of processing isnovel because it allows the sidewall passivation on the dielectric 640which forms as a by-product of the RIE processing to be removed beforethe fragile stencil pattern is formed. The sidewall passivation cleaningrequires vigorous agitation that would destroy a free standing stencilpattern. The remaining silicon layer 642 can be removed by gentlesilicon selective wet etch.

Stepper Contact Printer (SCP)

Pattern generation tools made from MDI circuit membranes can be used topattern an appropriately sensitive resist film deposited on a substrate.MDI pattern generation tools refer to MDI circuit membrane applicationssuch as the SLV, SPV and MLV lithographic tools, and the moreconventional fixed pattern masks made on a low stress dielectricmembrane with appropriate optical transmission characteristics (seeabove). A MDI fixed pattern mask is a single layer metallization patternformed on a MDI dielectric membrane with one of the MDI fabricationmethods presented above. A method of alignment to an existing image on asubstrate through electrical inductive coupling is presented below; itis also incorporated into the MDI circuit membrane fixed patterngeneration tool.

Contact printing methods are inexpensive and can provide nearlyunlimited lithographic feature size imaging capability. However, the useof thick mask plates for image transfer to the resist on the substratemust come in contact with the resist to achieve the best image results.This contact is brought about by the application of strong vacuum forcesthat bend the rigid mask plate into conformal contact with thesubstrate. The result of applying such strong vacuum forces often causessmall particles of the resist to adhere to the rigid mask which in turnrequires the mask to be cleaned or, if undetected, prevents correctimage transfer by nonconformal contact to the substrate or an imagingdefect during subsequent exposures. Proximity printing is similar tocontact printing except the mask is not brought into contact but is heldseveral micrometers over the substrate called the proximity gap. Theeffectiveness of proximity printing to produce small feature size imagesis directly proportional to the size of the proximity gap. Further,contact printing has been limited to printing a complete substrate inone exposure instead of smaller portions of the substrate as is the casewith conventional stepper projection lithography equipment. The time topull vacuum during the contact printing process is lengthy, andtherefore, making several contact print exposures on a single substratewould be prohibitive in time without consideration to the compoundedmask cleanliness challenge and the mechanical complexity of vacuumapplication to uneven or overlapping ridge mask and substrate plateduring contact.

FIG. 30 shows a cross-section of a stepping contact lithographicexposure equipment that combines the well known contact or proximitylithography methods with a stepper mechanical motion and the patterngeneration tools and alignment means of MDI circuit membranes presentedherein. The substrate (workpiece) 662 is held by a conventional vacuumchuck (not shown) and is positioned by conventional mechanical andpiezoelectric motion control mechanisms (not shown). The MDI patterngeneration tool 660 is held over the substrate 662 a distance "d" ofless than 25 μm and rough alignment over a portion of the substrate 662is made by conventional optical means. The center of the MDI patterngeneration tool 660 is then extended by applying a small fluid pressureP of less than 100 g per square cm to lower the circuit membrane into adesired proximity distance position shown by dotted line 668 orconformal contact with the substrate 662. Final precision alignment ofthe pattern generation tool 660 to the substrate is accomplished by theinductive coupling (electromagnetic) means disclosed below. The low massand elastic nature of the MDI circuit membrane allows the patterngeneration tool to be rapidly brought in and out of contact with thesubstrate. Once an exposure is complete, the pattern generation tool isbrought out of contact with the substrate and stepped (moved) undercomputer control to the next area on the substrate to be exposed. Alsoshown are circuit membrane frame 672 and a lithography equipment fixture676 for holding the pattern generation tool 660, 672.

Electro-Magnetic Lithoaraphic Alignment Method

The lithographic pattern generation tools disclosed herein as the SLV,SPV, MLV, and fixed masks are capable of pattern generation withCritical Dimensions (CDs) or minimum pattern feature sizes of less than50 nm. A fixed pattern generation tool, commonly referred to as a mask,can be made with the MDI circuit membrane process (as disclosed above)with the capability of pattern generation CDs directly proportional tothe wavelength of the exposure source. Potential exposure sources are V,DUW, E-Beam and particle beam. (Fixed masks made with the MDI processare circuit membranes of typically less than 4 μm thickness and with asingle layer of patterned material to be used as in conventionallithographic masks. Alternately, the MDI circuit membrane can be astencil pattern, where the pattern to be generated is represented in thecircuit membrane as trenches that go completely through the membrane.Fixed pattern stencil membranes are required for E-Beam or particle beamlithographic process, where the exposure source must physically passthrough the mask to be imaged (patterned). Prototype particle beamstencil masks have been developed by Ion Microfabrication Systems.)

The utility of the CD generation capability of a pattern generation toolfor the manufacture of ICs, however, is dependent on the ability toalign or register multiple overlaying patterns. Accepted patternalignment tolerances used presently in the semiconductor industry aretypically ±25% of the CD value for a set of overlaying patterns.

Alignment for pattern generation with CDs of approximately 1 μm orgreater can be done with optical microscopes by overlaying or matchingalignment patterns on the substrate with corresponding patterns on thesurface of the pattern generation tool. The microscope can view throughthe pattern generation tool, since it is typically made from transparentdielectric materials.

The preferred embodiments of the SLV, SPV, and MLV are to provide thecapability to generate patterns that have critical dimensions of lessthan 100 nm. Such patterns require alignment methods capable ofachieving pattern registration of less than ±25 nm. Optical alignmentmethods presently available cannot achieve such critical dimensions. TheMDI pattern generation tools achieve alignment for critical dimensionsof less than 100 nm through the use of electromagnetic proximitysensing. FIG. 28a shows an example of a PGT alignment coil 494. Thismetal coil 494 is fabricated by patterning a thin metal film (typicallyless than 1 μm in thickness) on the surface of the substrate beingpatterned (i.e, on the wafer). The placement of the coil patterns 494 onthe substrate is typically along two adjoining sides of the area to beexposed or patterned. Coil patterns can also be placed within the areaof the substrate to be patterned; they are not restricted to the edge orsides of the pattern area. The coil patterns are typically less than 800μm by 100 μm. The electrode contact pads 496, 498 of the coil area 494are typically 50 μm by 50 μm in area. A similar pattern 494corresponding to structure 430 in FIG. 24 is on the surface of thepattern generation tool membrane (not shown) plus two electrode probepoints that extend down from the surface of the circuit membrane of thepattern generation tool. These pattern generation tool probe points makecontact with the electrode pads associated with the substrate coil 494,and electrical signals are generated in the substrate coil 494. Itshould be apparent, though less convenient, that signals can be suppliedto the substrate by connections from the edge of the substrate and notdirectly from the pattern generation tool.

The signals from the substrate coil are sensed by the correspondingalignment coil in the pattern generation tool. The pattern generationtool or the substrate are then moved in an X-Y and angular manner untilpredefined electrical parameter settings are achieved via the signalssensed from the substrate coil. This is possible due to the closeproximity of the substrate and the pattern generation tool, and theintegrated control logic electronics of the pattern generation tool.Alignment sensing accuracies smaller than 10 nm can be achieved in thismanner. The electrode pads of the substrate coil typically havedimensions of less than 2 mils on a side, and the probe point electrodesthat extend from the pattern generation tool to make contact with thesubstrate coil pads are typically less than 12 μm in diameter andheight. A fluid pressure applied to the back side of the patterngeneration tool may be used to bring the surface of the patterngeneration tool in near contact (proximity) or full contact with thesurface of the substrate during the alignment process.

Alternately, FIG. 28b shows an alignment coil 495 that can be placed onthe substrate for alignment sensing by the pattern generation tool. Asignal is induced in one side 495a of the coil by a corresponding coilin the pattern generation tool. This signal is then sensed at the otherend 495b of the coil by a second coil in the pattern generation tool.Alignment is achieved in the same manner as discussed above.

Inexpensive piezoelectric motion control devices are commerciallyavailable from Burleigh Instruments, Inc. (Fisher, N.Y.) thatdemonstrate motion control capability of less than one Angstrom; suchpiezoelectric precision motion has been demonstrated in the AFM (AtomicForce Microscope) equipment where motion control of less than oneAngstrom is required.

MDI Circuit Membrane Flat Panel Display

Much of the cost of manufacturing a flat panel display (withoutconsideration for yield loss) is the cost of the substrate on which itis fabricated. The substrate must meet requirements of flatness imposedby lithography tools and high temperatures imposed by variousfabrication steps.

An MDI circuit membrane flat panel display is formed on a rigidoptically flat reusable substrate such as fused silica or fused quartz.Other substrates can be used to meet the requirements imposed by thefabrication processing steps. A release agent such as KBr or KBO₂ isdeposited onto the substrate and then a dielectric layer or layers aredeposited to seal the release agent deposited film (as described above).The release agent is required to have a working temperature higher thanthe highest temperature of the various fabrication process stepsemployed and be readily soluble in a solvent that is not detrimental tothe finished display device. As an example, KBr has a melting point of734° C. and is readily soluble in DI water.

The flat panel display is conventionally composed of rows and columns ofpixel elements which generate the gray shades or the RGB (Red, Green,Blue) colors of the display. The operation of the pixels results in thegeneration of the image on the display. The pixels of the display can bemade with conventional active matrix LCD technology. The preferredembodiment of the MDI circuit membrane display uses redundant circuitdevices at each pixel and the above referenced fine grain testingtechnology to correct for circuit fabrication defects. Secondly, the MDIcircuit membrane display makes use of electro luminescent phosphors,similar to the phosphors used in conventional TV and industrialmonitors. The phosphors are applied after the completion of thefabrication of the circuitry of the display. The display circuitry isthen used in the final manufacturing process steps to cause theselective deposition of the RGB phosphors to their respective RGBelectrodes. Additionally, the multiple chip module interconnect circuitmembrane technology disclosed above also is employed to bond ICs at theedges of the display panel and at any desired location on the back ofthe panel. The ability to attach ICs is of significant importance inlessening the complexity of the design and manufacturing of the display.ICs bonded to the back of the display do not interfere with or obscurethe visual operation of the phosphor based display, because the displayis not back lit as is the case with LCD displays, but generates its ownluminescence.

FIG. 31a shows the cross-section of a MDI circuit membrane display 700while still attached to a reusable quartz fabrication substrate 702 withintervening release agent 703. The control logic circuit 704-1, 704-2,704-3 for each RGB pixel has been fabricated from a-Si or polysiliconthin film transistors (TFTs), and electrophoretic plating electrodes708-1, 708-2, 708-3 (pads) have been formed as a final metallizationprocess step over each RGB pixel logic circuit 704. The platingelectrodes 708-1, 708-2, 708-3, one for each RGB phosphor of a pixel,can be selectively addressed and a specific plating potential applied.At the end of each row and column of pixels are bonding pads 710 for ICs(die). The ICs provide control and memory logic for turning individualpixels on or off, therefore generating a display image. It should benoted that the control and/or memory logic 712, 713 (see FIG. 31b) couldalso be fabricated as part of the MDI circuit membrane 700 just as theindividual pixel control circuits 704-1, 704-2, 704-3 have been. This isa design/manufacturing option, and would replace most or all of thebonded die. All fabrication processing steps of the MDI circuit membraneare established techniques.

FIG. 31b shows in cross section the MDI circuit membrane display 700attached to a support frame 718 and released from the reusablefabrication substrate. The support frame 718 is bonded to the MDIcircuit membrane 700 by an anodic method as one technique, and then theMDI circuit membrane 700 is scribed along the side of the fabricationsubstrate 702. This exposes the release agent 703 of FIG. 31a. Thedisplay assembly is then placed in a solvent that activates the releaseagent 703, causing the display to be freed from the reusable fabricationsubstrate 702. Optionally, control and memory logic die 713 are attachedto the display backside. These IC 713 connect to pixels along row orcolumn interconnect metallization.

FIG. 31c shows the display after electro-phoretic plating of theindividual RGB phosphors 708-1, 708-2, 708-3. Each RGB phosphor isplated separately by immersion of the display in a plating solution of aspecific phosphor, and then using the control and pixel logic 704-1,704-2, 704-3 of the display to select appropriate pixel elements (eitherRed or Green or Blue) and apply the desired plating electricalpotential.

The substrate used to fabricate the flat screen display could also be aMDI membrane with a semiconductor device layer. Using a semiconductorbased MDI membrane as the substrate to fabricate the flat screen displaywill not require any substantial difference in processing steps,however, due to the limited size of available silicon wafers (presently8 inches) and the circular shape, the use of a semiconductor wafer asthe starting substrate greatly limits the maximum size of the display.

Three Dimensional IC Structures

Three dimensional (3D) IC structures can be formed from MDI circuitmembranes. This is a novel capability of the MDI IC membrane. The lowstress dielectrics of silicon dioxide or silicon nitride can be formedto withstand working temperatures in excess of 400° C. This high workingtemperature capability allows the use of anodic or thermal (quartz toquartz or silicon to quartz) bonding procedures that are typically hightemperature processes. The dielectric circuit membranes are opticallytransparent and thin, allowing the circuit membranes to be aligned veryaccurately prior to bonding. As shown in FIG. 32a, the semiconductordevices 730-1, 730-2, 730-3 that are embedded in a first MDI circuitmembrane 732 can have interconnect metallization 736-1, 736-2, 736-3applied to both sides of the membrane 732 completing all intra-membraneinterconnections. Also shown are opposed semiconductor device gateelectrodes 742-1, 742-2, 742-3. FIG. 32a shows the two MDI circuitmembranes or ICs 732, 746 prior to bonding. This allows the circuitmembranes to be coated as necessary for the forming of the anodic orthermal membrane-to-membrane bond 752 of circuit membrane 732 to thesecond membrane 746 which includes similarly semiconductor devices748-1, 748-2, 748-3, and interconnect metallization 750-1, 750-2, 750-3.After two membranes 732, 746 are bonded as shown in FIG. 32b, onlyvertical interconnect metallization 754-1, 754-2 can be employed toroute signals from lower membrane layer 746.

MDI circuit membranes 732, 746 are aligned using established opticalalignment mark techniques and anodically or thermally bonded 752 to eachother. The circuit membranes 732, 746 that are anodically or thermallybonded are complete of all necessary semiconductor device processingsteps, and only require the completion of vertical interconnectmetallization 754-1, 754-2. The final interconnect metallization 754-1,754-2 is completed by etching vias 756-1, 756-2 through the last circuitmembrane 732 to be bonded to electrical contacts on the circuit membrane746 onto which it was bonded. Well known metal deposition and patterningprocess steps are applied. If additional circuit membrane ICs are to bebonded to a 3D IC structure, a dielectric coating is applied to the 3Dstructure to isolate any metallization as a necessary requirement to asubsequent bonding step.

Anodic bonding 752 is a glass-to-glass process. The MDI circuitmembranes 732, 746 are each provided with a 1 μm thick or greater finaldeposition of silicon dioxide to form the bond 752 prior to the anodicbonding process step. Anodic processing is well known and there arenumerous such techniques used in conventional glass plate manufacturing.

Thermal (fusion) bonding of silicon to quartz or glass substrates is anestablished technique. It can be used instead of anodic bonding methodsto bond MDI circuit membranes.

The ability to use optical data transmission as an interconnect means(instead of metallization interconnect) between MDI circuit membranelayers 732, 746 is a direct consequence of the method of fabricating the3D IC structure. Wave guides (not shown) between optical transceivercomponents can be formed from vias by bonding vias (openings)corresponding to the various optical transceiver devices on circuitmembranes. The normally high transmissive nature of the circuitmembranes and the short layer-to-layer distances between the circuitmembranes allow integrated optical semiconductor devices to provide thecommunication function through vias or directly through certaindielectric materials.

Integrated circuits formed on a MDI circuit membrane can be cut from thecircuit membrane in much the same manner as ICs or die are presently cutfrom a rigid wafer substrate in order to be packaged. This is possibledue to the net low stress of the circuit membrane. FIGS. 32c and d showdie cut from a circuit membrane and bonded onto a rigid substrate suchas a silicon wafer, quartz, glass, metal, etc. In the case of a metalrigid substrate, dielectric and interconnect layers may be added toprovide a better bonding surface for the membrane IC or to provideinterconnection from bond pads on the IC to bond out pads at the edge ofthe metal substrate. The use of a metal substrate to attach the membraneIC provides clear advantages, if not an optimal means, for thermalcooling of the IC.

Once the circuit membrane substrate or wafer is cut into dice in themanner which is present practice, a pick and place bonding tool can pickup the die with slight vacuum pressure, align it over the substrate ontowhich it is to be bonded and bond it to the substrate. FIGS. 32c and deach show stocked membrane die 764-a, 764-b, 764-c bonded to a rigidsubstrate. Alternately, the membrane die 764-a, 764-b, 764-c can bebonded with compressive, anodic or fusion (thermal) techniques to acircuit membrane (not shown), such as a MCM interconnect circuitmembrane.

FIG. 32c shows a 3D circuit structure of membrane dice (ICs) 764-a,764-b, 764-c bonded by compression technique to a rigid substrate 770.This is accomplished by aligning metal pads 772-1, 1,772-2, . . . ,772-k, . . . , 772-x on the rigid substrate 770 and metal pads 776-1,776-2, . . . , 776-w on the membrane die 764-a, . . . , 764-c and thenapplying temperature and pressure (as explained previously). Also shownare bonding-out pads 778-1, 778-2. The surfaces are bonded together bythe metal pads which also serve as circuit interconnect and thermalrelief paths.

FIG. 32d shows a 3D circuit structure of membrane dice 764-a, 764-b,764-c bonded by anodic or fusion (thermal) techniques to a substrate770. The handling of the individual die 764-a, 764-b, 764-c can be donein a manner similar to that described above, and the bonding andinterconnect completion methods are also as described above for bondingthe circuit membranes together.

The benefits of such die assembly practice are dramatically increasedcircuit device densities and low thermal mass for efficient cooling. Thehandling equipment for making such a circuit assembly is presentlyavailable.

This disclosure is illustrative and not limiting; further modificationswill be apparent to one skilled in the art in light of this disclosureand the appended claims.

I claim:
 1. A method of making a dielectrically isolated integratedcircuit comprising the steps of:providing a substrate having a principalsurface; forming an etch barrier layer in the substrate parallel to theprincipal surface; forming semiconductor devices on the principalsurface; after forming the semiconductor devices, depositing a lowstress insulating membrane over the semiconductor devices, wherein saidinsulating membrane has a net surface tensile stress level 2-100 timesless than a fracture strength of the insulating membrane material; andetching away to the etch barrier layer a portion of the substrate from abackside of the substrate opposite the principal surface.
 2. The methodof claim 1, further comprising the steps of:forming an epitaxial layeron the principal surface; and forming the semiconductor devices in theepitaxial layer.
 3. The method of claim 1, further comprising the stepof forming a trench in the substrate isolating at least one of thesemiconductor devices from the others.
 4. The method of claim 1, whereinthe etch barrier layer is formed lying less than 50 μm below theprincipal surface.
 5. The method of claim 1, further comprising the stepof forming conductors in the insulating membrane for interconnecting thesemiconductor devices.
 6. The method of claim 1, wherein the step ofetching away comprises the step of leaving an edge portion of thesubstrate unetched, thereby supporting a central etched away portion ofthe substrate.
 7. The method of claim 1, further comprising the step ofattaching an annular support ring to an edge portion of the substrate.8. The method of claim 1, wherein the step of depositing includes thestep of supplying SiH₄ gas, N₂ O gas, and N₂ gas at a temperature ofabout 400° C.
 9. The method of claim 1, wherein the step of depositingincludes the step of supplying SiH₄ gas, NH₃ gas, and N₂ gas at atemperature of about 400° C.
 10. The method of claim 1, wherein the stepof forming an etch barrier layer comprises:implanting the substrate witha material to form a lightly doped etch stop barrier layer, wherein theimplanted material is one of oxygen or nitrogen.
 11. The method of claim1, wherein the step of providing comprises the steps of:providing asemiconductor substrate; doping the semiconductor substrate to form anetch stop layer in the substrate; epitaxially growing agermanium-semiconductor material layer on a surface of the substrate;and etching the substrate to remove the germanium-semiconductor materiallayer and the etch stop layer, thereby forming the semiconductor layer.12. The method of claim 1, further comprising the steps of:forming anadditional thickness of low stress insulating material over themembrane; and forming a plurality of recesses in the additionalthickness.
 13. The method of claim 1, wherein the step of formingsemiconductor devices separates the semiconductor devices such that,after the step of etching away, each semiconductor device iselectrically isolated by the insulating membrane from each of the othersemiconductor devices.
 14. The method of claim 1, wherein the step offorming semiconductor devices separates the semiconductor devices fromthe substrate such that each semiconductor device is electricallyisolated from the substrate by the insulating membrane.
 15. The methodof claim 1, wherein the membrane is selected from the group consistingof silicon dioxide and silicon nitride.
 16. The method of claim 1,wherein the insulating membrane has a thickness in the range of about 2to about 15 micrometers.
 17. The method of claim 1, further comprisingthe step of etching a recess in a back surface of the substrate.
 18. Themethod of claim 1, wherein the insulating membrane is formed of silicondioxide.
 19. The method of claim 1, wherein the insulating membrane isformed of silicon nitride.
 20. The method of claim 1, wherein the stepof forming an etch barrier layer comprises epitaxial processing.
 21. Themethod of claim 20, wherein the step of epitaxial processingcomprises:forming a heavily doped boron layer; and epitaxially growing aSiGe layer of less than 25% Ge on either side of the boron layer. 22.The method of claim 1, wherein the substrate comprises polycrystallinesilicon.
 23. The method of claim 1, wherein the substrate comprises amonocrystalline material.
 24. The method of claim 1, wherein thesubstrate comprises an amorphous material.
 25. The method of claim 6,wherein a width of the edge portion is less than about 1 cm.
 26. Themethod of claim 7, wherein the support ring has a thickness in the rangeof about 25 to about 100 mils.
 27. The method of claim 8, wherein thestep of depositing is performed at a pressure of about 1.8 torr.
 28. Themethod of claim 9, wherein the step of depositing is performed at apressure of about 2.3 torr.
 29. The method of claim 8, wherein the stepof depositing includes applying both high and low radio frequency power.30. The method of claim 9, wherein the step of depositing includesapplying both high and low radio frequency power.
 31. The method ofclaim 1, wherein the etch barrier layer is formed with a materialselected from the group consisting of boron, oxygen, nitrogen, andporous silicon.
 32. The method of claim 12, wherein a depth of therecesses is about 75% of the additional thickness.
 33. The method ofclaim 32, wherein a width of the recesses is about two to three timesthe depth of the recesses.
 34. A method of making a dielectricallyisolated integrated circuit comprising the steps of:providing asubstrate having a principal surface; forming an etch barrier layer inthe substrate parallel to the principal surface; forming semiconductordevices on the principal surface; after forming the semiconductordevices, depositing a low stress insulating membrane over thesemiconductor devices; etching away to the etch barrier layer a portionof the substrate from a backside of the substrate opposite the principalsurface; and attaching an annular support ring to an edge portion of thesubstrate.
 35. The method of claim 34 wherein the support ring has athickness in the range of about 25 to about 100 mils.
 36. A method ofmaking a dielectrically isolated integrated circuit comprising the stepsof:providing a substrate having a principal surface; forming an etchbarrier layer in the substrate parallel to the principal surface byforming a heavily doped boron layer and epitaxially growing a SiGe layerof less than 25% Ge on either side of the boron layer; formingsemiconductor devices on the principal surface; after forming thesemiconductor devices, depositing a low stress insulating membrane overthe semiconductor devices; and etching away to the etch barrier layer aportion of the substrate from a backside of the substrate opposite theprincipal surface.
 37. A method of making a dielectrically isolatedintegrated circuit comprising the steps of:providing a substrate havinga principal surface, wherein the substrate comprises polycrystallinesilicon; forming an etch barrier layer in the substrate parallel to theprincipal surface; forming semiconductor devices on the principalsurface; after forming the semiconductor devices, depositing a lowstress insulating membrane over the semiconductor devices; and etchingaway to the etch barrier layer a portion of the substrate from abackside of the substrate opposite the principal surface.
 38. A methodof making a dielectrically isolated integrated circuit comprising thesteps of:providing a substrate having a principal surface, wherein thesubstrate comprises an amorphous material; forming an etch barrier layerin the substrate parallel to the principal surface; formingsemiconductor devices on the principal surface; after forming thesemiconductor devices, depositing a low stress insulating membrane overthe semiconductor devices; and etching away to the etch barrier layer aportion of the substrate from a backside of the substrate opposite theprincipal surface.
 39. A method of making a dielectrically isolatedintegrated circuit comprising the steps of:providing a substrate havinga principal surface; forming an etch barrier layer in the substrateparallel to the principal surface; forming semiconductor devices on theprincipal surface; after forming the semiconductor devices, depositing alow stress insulating membrane over the semiconductor devices; etchingaway to the etch barrier layer a portion of the substrate from abackside of the substrate opposite the principal surface; forming anadditional thickness of low stress insulating material over themembrane; and forming a plurality of recesses in the additionalthickness, wherein a depth of the recesses is about 75% of theadditional thickness and wherein a width of the recesses is about two tothree times the depth of the recesses.
 40. A method of making adielectrically isolated integrated circuit comprising the stepsof:providing a substrate having a principal surface; forming an etchbarrier layer in the substrate parallel to the principal surface;forming semiconductor devices on the principal surface; after formingthe semiconductor devices, depositing a low stress insulating membraneover the semiconductor devices, wherein said insulating membrane isdeposited with a material selected from the group consisting of silicondioxide and silicon nitride and has a surface stress level of less than8×10⁸ dynes/cm² in tension; and etching away to the etch barrier layer aportion of the substrate from a backside of the substrate opposite theprincipal surface.
 41. The method of claim 40, further comprising thesteps of:forming an epitaxial layer on the principal surface; andforming the semiconductor devices in the epitaxial layer.
 42. The methodof claim 40, further comprising the step of forming a trench in thesubstrate isolating at least one of the semiconductor devices from theothers.
 43. The method of claim 40, wherein the etch barrier layer isformed lying less than 50 μm below the principal surface.
 44. The methodof claim 40, further comprising the step of forming conductors in theinsulating membrane for interconnecting the semiconductor devices. 45.The method of claim 40, wherein the step of etching away comprises thestep of leaving an edge portion of the substrate unetched, therebysupporting a central etched away portion of the substrate.
 46. Themethod of claim 40, further comprising the step of attaching an annularsupport ring to an edge portion of the substrate.
 47. The method ofclaim 40, wherein the step of depositing includes the step of supplyingSiH₄ gas, N₂ O gas, and N₂ gas at a temperature of about 400° C.
 48. Themethod of claim 40, wherein the step of depositing includes the step ofsupplying SiH₄ gas, NH₃ gas, and N₂ gas at a temperature of about 400°C.
 49. The method of claim 40, wherein the step of forming an etchbarrier layer comprises:implanting the substrate with a material to forma lightly doped etch stop barrier layer, wherein the implanted materialis one of oxygen or nitrogen.
 50. The method of claim 40, wherein thestep of providing comprises the steps of:providing a semiconductorsubstrate; doping the semiconductor substrate to form an etch stop layerin the substrate; epitaxially growing a germanium-semiconductor materiallayer on a surface of the substrate; and etching the substrate to removethe germanium-semiconductor material layer and the etch stop layer,thereby forming the semiconductor layer.
 51. The method of claim 40,further comprising the steps of:forming an additional thickness of lowstress insulating material over the membrane; and forming a plurality ofrecesses in the additional thickness.
 52. The method of claim 40,wherein the step of forming semiconductor devices separates thesemiconductor devices such that, after the step of etching away, eachsemiconductor device is electrically isolated by the insulating membranefrom each of the other semiconductor devices.
 53. The method of claim40, wherein the step of forming semiconductor devices separates thesemiconductor devices from the substrate such that each semiconductordevice is electrically isolated from the substrate by the insulatingmembrane.
 54. The method of claim 40, wherein the membrane is selectedfrom the group consisting of silicon dioxide and silicon nitride. 55.The method of claim 40, wherein the insulating membrane has a thicknessin the range of about 2 to about 15 micrometers.
 56. The method of claim40, further comprising the step of etching a recess in a back surface ofthe substrate.
 57. The method of claim 40, wherein the insulatingmembrane is formed of silicon dioxide.
 58. The method of claim 40,wherein the insulating membrane is formed of silicon nitride.
 59. Themethod of claim 40, wherein the step of forming an etch barrier layercomprises epitaxial processing.
 60. The method of claim 59, wherein thestep of epitaxial processing comprises:forming a heavily doped boronlayer; and epitaxially growing a SiGe layer of less than 25% Ge oneither side of the boron layer.
 61. The method of claim 40, wherein thesubstrate comprises polycrystalline silicon.
 62. The method of claim 40,wherein the substrate comprises a monocrystalline material.
 63. Themethod of claim 40, wherein the substrate comprises an amorphousmaterial.
 64. The method of claim 45, wherein a width of the edgeportion is less than about 1 cm.
 65. The method of claim 46, wherein thesupport ring has a thickness in the range of about 25 to about 100 mils.66. The method of claim 47, wherein the step of depositing is performedat a pressure of about 1.8 torr.
 67. The method of claim 48, wherein thestep of depositing is performed at a pressure of about 2.3 torr.
 68. Themethod of claim 47, wherein the step of depositing includes applyingboth high and low radio frequency power.
 69. The method of claim 48,wherein the step of depositing includes applying both high and low radiofrequency power.
 70. The method of claim 40, wherein the etch barrierlayer is formed with a material selected from the group consisting ofboron, oxygen, nitrogen, and porous silicon.
 71. The method of claim 51,wherein a depth of the recesses is about 75% of the additionalthickness.
 72. The method of claim 71, wherein a width of the recessesis about two to three times the depth of the recesses.
 73. The method ofclaim 1 wherein the substrate has an area greater than 1 square cm. 74.The method of claim 5 wherein the conductors are formed in the membraneboth above and below the semiconductor devices.
 75. The method of claim1 further comprising at least one additional processing step performedat a temperature in excess of 400° C.
 76. The method of claim 75 whereinsaid additional processing step comprises implant activation annealing.77. The method of claim 40, wherein the substrate has an area greaterthan 1 square cm.
 78. The method of claim 44 wherein the conductors areformed in the membrane both above and below the semiconductor devices.79. The method of claim 40 further comprising at least one additionalprocessing step performed at a temperature in excess of 400° C.
 80. Themethod of claim 79 wherein said additional processing step comprisesimplant activation annealing.
 81. A method of making a dielectricallyisolated integrated circuit comprising the steps of:providing asubstrate having a principal surface; forming an etch barrier layer inthe substrate parallel to the principal surface; forming semiconductordevices on the principal surface; after forming the semiconductordevices, depositing a low-stress insulating membrane over thesemiconductor devices; and etching away to the etch barrier layer aportion of the substrate from the backside of the substrate opposite theprincipal surface; wherein structural integrity of a substantial portionof the dielectrically isolated integrated circuit is derived primarilyfrom the low stress insulated membrane.